SQL FAQ

Oracle SQL FAQ:

What is SQL and where does it come from?

Structured Query Language (SQL) is a language that provides an interface to relational database systems. The proper pronunciation of SQL, and the preferred pronunciation within Oracle Corp, is "sequel" and not "ess cue ell".

SQL was developed by IBM in the 1970s for use in System R, and is a de facto standard, as well as an ISO and ANSI standard.

In common usage SQL also encompasses DML (Data Manipulation Language), for INSERTs, UPDATEs, DELETEs and DDL (Data Definition Language), used for creating and modifying tables and other database structures.

The development of SQL is governed by standards. A major revision to the SQL standard was completed in 1992, called SQL2. SQL3 support object extensions and are (partially?) implemented in Oracle8 and 9i.

Example SQL statements:

CREATE TABLE table1 (column1 NUMBER, column2 VARCHAR2(30));

INSERT INTO table1 VALUES (1, 'XYZ');

SELECT * FROM table1 WHERE column2 = 'XYZ';

What are the difference between DDL, DML and DCL commands?

DDL - Data Definition Language: statements used to define the database structure or schema. Some examples:

• CREATE - to create objects in the database
• ALTER - alters the structure of the database
• DROP - delete objects from the database
• TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
• COMMENT - add comments to the data dictionary
• RENAME - rename an object

DML - Data Manipulation Language: statements used for managing data within schema objects. Some examples:

• SELECT - retrieve data from the a database
• INSERT - insert data into a table
• UPDATE - updates existing data within a table
• DELETE - deletes all records from a table, the space for the records remain
• MERGE - UPSERT operation (insert or update)
• CALL - call a PL/SQL or Java subprogram
• EXPLAIN PLAN - explain access path to the data
• LOCK TABLE - controls concurrency

DCL - Data Control Language. Some examples:

• GRANT - gives user's access privileges to database
• REVOKE - withdraw access privileges given with the GRANT command

TCL - Transaction Control: statements used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.

• COMMIT - save work done
• SAVEPOINT - identify a point in a transaction to which you can later roll back
• ROLLBACK - undo the modification I made since the last COMMIT
• SET TRANSACTION - Change transaction options like isolation level and what rollback segment to use
• SET ROLE - set the current active roles

DML are not auto-commit. i.e. you can roll-back the operations, but DDL are auto-commit

Difference between TRUNCATE, DELETE and DROP commands?

The DELETE command is used to remove some or all rows from a table. A WHERE clause can be used to only remove some rows. If no WHERE condition is specified, all rows will be removed. After performing a DELETE operation you need to COMMIT or ROLLBACK the transaction to make the change permanent or to undo it. Note that this operation will cause all DELETE triggers on the table to fire.

SQL> SELECT COUNT(*) FROM emp;
  COUNT(*)
----------
        14

SQL> DELETE FROM emp WHERE job = 'CLERK';
4 rows deleted.

SQL> COMMIT;
Commit complete.

SQL> SELECT COUNT(*) FROM emp;
  COUNT(*)
----------
        10

TRUNCATE removes all rows from a table. The operation cannot be rolled back and no triggers will be fired. As such, TRUNCATE is faster and doesn't use as much undo space as a DELETE.

SQL> TRUNCATE TABLE emp;
Table truncated.

SQL> SELECT COUNT(*) FROM emp;

  COUNT(*)
----------
         0

The DROP command removes a table from the database. All the tables' rows, indexes and privileges will also be removed. No DML triggers will be fired. The operation cannot be rolled back.

SQL> DROP TABLE emp;
Table dropped.

SQL> SELECT * FROM emp;
SELECT * FROM emp
              *
ERROR at line 1:
ORA-00942: table or view does not exist

DROP and TRUNCATE are DDL commands, whereas DELETE is a DML command. Therefore DELETE operations can be rolled back (undone), while DROP and TRUNCATE operations cannot be rolled back.

From Oracle 10g a table can be "undropped". Example:

SQL> FLASHBACK TABLE emp TO BEFORE DROP;
Flashback complete.

PS: DELETE will not free up used space within a table. This means that repeated DELETE commands will severely fragment the table and queries will have to navigate this "free space" in order to retrieve rows.

How does one drop all tables in a given schema?

Use a script like this to generate the required DROP statements. You will be prompted for the schema name.

undefine owner
set pages 0
set lines 300
set heading off
spool /tmp/drop_statements.sql
select 'drop table '||owner||'.'||table_name||' cascade constraints purge;' 
  from dba_tables
 where owner = upper('&&owner')
union all
select 'drop '||object_type||' '||owner||'.'||object_name||';'
  from dba_objects
 where object_type not in ('TABLE','INDEX','PACKAGE BODY','TRIGGER','LOB')
   and object_type not like '%LINK%'
   and object_type not like '%PARTITION%'
   and owner = upper('&&owner')
 order by 1;
spool off

Now inspect the file and run it to do the actual dropping:

@/tmp/drop_statements.sql
PROMPT Count of remaining objects =
select object_type,count(*) from dba_objects where owner = upper('&&owner') group by object_type;

How does one escape special characters when writing SQL queries?

Escape quotes

Use two quotes for every one displayed. Examples:

SQL> SELECT 'Frank''s Oracle site' AS text FROM DUAL;
 TEXT
 --------------------
 Franks's Oracle site

 SQL> SELECT 'A ''quoted'' word.' AS text FROM DUAL;
 TEXT
 ----------------
 A 'quoted' word.

 SQL> SELECT 'A ''''double quoted'''' word.' AS text FROM DUAL;
 TEXT
 -------------------------
 A ''double quoted'' word.

Use Q expression:

SQL> SELECT q'[Frank's Oracle site]' AS text FROM DUAL;
 TEXT
 -------------------
 Frank's Oracle site

 SQL> SELECT q'/A 'quoted' word./' AS text FROM DUAL;
 TEXT
 ----------------
 A 'quoted' word.

 SQL> SELECT q'{A ''double quoted'' word.}' AS text FROM DUAL;
 TEXT
 -------------------------
 A ''double quoted'' word.

Escape wildcard characters

The LIKE keyword allows for string searches. The '_' wild card character is used to match exactly one character, while '%' is used to match zero or more occurrences of any characters. These characters can be escaped in SQL. Examples:

SELECT name FROM emp 
 WHERE id LIKE '%/_%' ESCAPE '/';

SELECT name FROM emp 
 WHERE id LIKE '%\%%' ESCAPE '\';

Escape ampersand (&) characters in SQL*Plus

When using SQL*Plus, the DEFINE setting can be changed to allow &'s (ampersands) to be used in text:

SET DEFINE ~
SELECT 'Laurel & Hardy' FROM dual;

Other methods:

Define an escape character:

SET ESCAPE '\'
SELECT '\&abc' FROM dual;

Don't scan for substitution variables:

SET SCAN OFF
SELECT '&ABC' x FROM dual;

Another way to escape the & would be to use concatenation, which would not require any SET commands -

SELECT 'Laurel ' || '&' || ' Hardy' FROM dual;

Can one select a random collection of rows from a table?

The following methods can be used to select a random collection of rows from a table:

The SAMPLE Clause

From Oracle 8i, the easiest way to randomly select rows from a table is to use the SAMPLE clause with a SELECT statement. Examples:

SELECT * FROM emp SAMPLE(10);

In the above example, Oracle is instructed to randomly return 10% of the rows in the table.

SELECT * FROM emp SAMPLE(5) BLOCKS;

This example will sample 5% of all formatted database blocks instead of rows.

This clause only works for single table queries on local tables. If you include the SAMPLE clause within a multi-table or remote query, you will get a parse error or "ORA-30561: SAMPLE option not allowed in statement with multiple table references". One way around this is to create an inline view on the driving table of the query with the SAMPLE clause. Example:

SELECT t1.dept, t2.emp
  FROM (SELECT * FROM dept SAMPLE(5)) t1,
       emp t2
 WHERE t1.dep_id = t2.dep_id;

If you examine the execution plan of a "Sample Table Scan", you should see a step like this:

TABLE ACCESS (SAMPLE) OF 'EMP' (TABLE)

ORDER BY dbms_random.value()

This method orders the data by a random column number. Example:

SQL> SELECT * FROM (SELECT ename
  2                   FROM emp
  3                  ORDER BY dbms_random.value())
  4   WHERE rownum <= 3;
ENAME
----------
WARD
MILLER
TURNER

The ORA_HASH() function

The following example retrieves a subset of the data in the emp table by specifying 3 buckets (0 to 2) and then returning the data from bucket 1:

SELECT * FROM emp WHERE ORA_HASH(empno, 2) = 1;

How does one eliminate duplicates rows from a table?

Choose one of the following queries to identify or remove duplicate rows from a table leaving only unique records in the table:

Method 1:

Delete all rowids that is BIGGER than the SMALLEST rowid value (for a given key):

SQL> DELETE FROM table_name A 
  2  WHERE ROWID > ( SELECT min(rowid) 
  3                  FROM table_name B
  4                  WHERE A.key_values = B.key_values );

Method 2:

This method is usually faster. However, remember to recreate all indexes, constraints, triggers, etc. on the table when done.

SQL> create table table_name2 as select distinct * from table_name1;
SQL> drop table table_name1;
SQL> rename table_name2 to table_name1;

Method 3:

SQL> DELETE FROM my_table t1
  2  WHERE EXISTS ( SELECT 'x' FROM my_table t2
  3                 WHERE t2.key_value1 = t1.key_value1
  4                   AND t2.key_value2 = t1.key_value2
  4                   AND t2.rowid      > t1.rowid );

Note: One can eliminate N^2 unnecessary operations by creating an index on the joined fields in the inner loop (no need to loop through the entire table on each pass by a record). This will speed-up the deletion process.

Note 2: If you are comparing NULL columns, use the NVL function. Remember that NULL is not equal to NULL. This should not be a problem as all key columns should be NOT NULL by definition.

Method 4:

This method collects the first row (order by rowid) for each key values and delete the rows that are not in this set:

SQL> DELETE FROM my_table t1
  1  WHERE rowid NOT IN ( SELECT min(rowid) 
  2                       FROM my_table t2
  3                       GROUP BY key_value1, key_value2 );

Note: IF key_value1 is null or key_value2 is null, this still works correctly.

How does one get the time difference between two date columns?

Oracle allows two date values to be subtracted from each other returning a numeric value indicating the number of days between the two dates (may be a fraction). This example will show how to relate it back to a time value.

Let's investigate some solutions. Test data:

SQL> CREATE TABLE dates (date1 DATE, date2 DATE);
Table created.
SQL>
SQL> INSERT INTO dates VALUES (SYSDATE, SYSDATE-1);
1 row created.
SQL> INSERT INTO dates VALUES (SYSDATE, SYSDATE-1/24);
1 row created.
SQL> INSERT INTO dates VALUES (SYSDATE, SYSDATE-1/60/24);
1 row created.
SQL> SELECT (date1 - date2) FROM dates;
DATE1-DATE2
-----------
          1
 .041666667
 .000694444

Solution 1

SQL> SELECT floor(((date1-date2)*24*60*60)/3600)
  2         || ' HOURS ' ||
  3         floor((((date1-date2)*24*60*60) -
  4         floor(((date1-date2)*24*60*60)/3600)*3600)/60)
  5         || ' MINUTES ' ||
  6         round((((date1-date2)*24*60*60) -
  7         floor(((date1-date2)*24*60*60)/3600)*3600 -
  8         (floor((((date1-date2)*24*60*60) -
  9         floor(((date1-date2)*24*60*60)/3600)*3600)/60)*60) ))
 10         || ' SECS ' time_difference
 11    FROM dates;
TIME_DIFFERENCE
--------------------------------------------------------------------------------
24 HOURS 0 MINUTES 0 SECS
1 HOURS 0 MINUTES 0 SECS
0 HOURS 1 MINUTES 0 SECS

Solution 2

If you don't want to go through the floor and ceiling maths, try this method:

SQL> SELECT to_number( to_char(to_date('1','J') +
  2         (date1 - date2), 'J') - 1)  days,
  3         to_char(to_date('00:00:00','HH24:MI:SS') +
  4         (date1 - date2), 'HH24:MI:SS') time
  5   FROM dates;
      DAYS TIME
---------- --------
         1 00:00:00
         0 01:00:00
         0 00:01:00

Solution 3

Here is a simpler method:

SQL> SELECT trunc(date1-date2) days,
  2     to_char(trunc(sysdate) + (date1 - date2), 
  3             'HH24 "Hours" MI "Minutes" SS "Seconds"') time
  4   FROM dates;
      DAYS TIME
---------- ------------------------------
         1 00 Hours 00 Minutes 00 Seconds
         0 01 Hours 00 Minutes 00 Seconds
         0 00 Hours 01 Minutes 00 Seconds

How does one add a day/hour/minute/second to a date value?

The SYSDATE pseudo-column shows the current system date and time. Adding 1 to SYSDATE will advance the date by 1 day. Use fractions to add hours, minutes or seconds to the date. Look at these examples:

SQL> select sysdate, sysdate+1/24, sysdate +1/1440, sysdate + 1/86400 from dual;
SYSDATE              SYSDATE+1/24         SYSDATE+1/1440       SYSDATE+1/86400
-------------------- -------------------- -------------------- --------------------
03-Jul-2002 08:32:12 03-Jul-2002 09:32:12 03-Jul-2002 08:33:12 03-Jul-2002 08:32:13

The following format is frequently used with Oracle Replication:

select sysdate NOW, sysdate+30/(24*60*60) NOW_PLUS_30_SECS from dual;
NOW                  NOW_PLUS_30_SECS
-------------------- --------------------
03-JUL-2005 16:47:23 03-JUL-2005 16:47:53

Here are a couple of examples:

Description	Date Expression
Now	SYSDATE
Tomorow/ next day	SYSDATE + 1
Seven days from now	SYSDATE + 7
One hour from now	SYSDATE + 1/24
Three hours from now	SYSDATE + 3/24
A half hour from now	SYSDATE + 1/48
10 minutes from now	SYSDATE + 10/1440
30 seconds from now	SYSDATE + 30/86400
Tomorrow at 12 midnight	TRUNC(SYSDATE + 1)
Tomorrow at 8 AM	TRUNC(SYSDATE + 1) + 8/24
Next Monday at 12:00 noon	NEXT_DAY(TRUNC(SYSDATE), 'MONDAY') + 12/24
First day of the month at 12 midnight	TRUNC(LAST_DAY(SYSDATE ) + 1)
The next Monday, Wednesday or Friday at 9 a.m	TRUNC(LEAST(NEXT_DAY(sysdate, 'MONDAY'), NEXT_DAY(sysdate, 'WEDNESDAY'), NEXT_DAY(sysdate, 'FRIDAY'))) + 9/24

How does one code a matrix/crosstab/pivot report in SQL?

Newbies frequently ask how one can display "rows as columns" or "columns as rows". Look at these example crosstab queries (also sometimes called transposed, matrix or pivot queries):

SELECT  *
  FROM  (SELECT job,
                sum(decode(deptno,10,sal)) DEPT10,
                sum(decode(deptno,20,sal)) DEPT20,
                sum(decode(deptno,30,sal)) DEPT30,
                sum(decode(deptno,40,sal)) DEPT40
           FROM scott.emp
       GROUP BY job)
ORDER BY 1;
JOB           DEPT10     DEPT20     DEPT30     DEPT40
--------- ---------- ---------- ---------- ----------
ANALYST                    6000
CLERK           1300       1900        950
MANAGER         2450       2975       2850
PRESIDENT       5000
SALESMAN                              5600

Here is the same query with some fancy headers and totals:

SQL> ttitle "Crosstab Report"
SQL> break on report;
SQL> compute sum of dept10 dept20 dept30 dept40 total on report;
SQL>
SQL> SELECT     *
  2    FROM     (SELECT job,
  3                  sum(decode(deptno,10,sal)) DEPT10,
  4                  sum(decode(deptno,20,sal)) DEPT20,
  5                  sum(decode(deptno,30,sal)) DEPT30,
  6                  sum(decode(deptno,40,sal)) DEPT40,
  7                  sum(sal)                   TOTAL
  8             FROM emp
  9            GROUP BY job)
 10  ORDER BY 1;

Mon Aug 23                                                             page    1
                                Crosstab Report

JOB           DEPT10     DEPT20     DEPT30     DEPT40      TOTAL
--------- ---------- ---------- ---------- ---------- ----------
ANALYST                    6000                             6000
CLERK           1300       1900        950                  4150
MANAGER         2450       2975       2850                  8275
PRESIDENT       5000                                        5000
SALESMAN                              5600                  5600
          ---------- ---------- ---------- ---------- ----------
sum             8750      10875       9400                 29025

Here's another variation on the theme:

SQL> SELECT DECODE(MOD(v.row#,3)
  2                 ,1, 'Number: '  ||deptno
  3                 ,2, 'Name: '    ||dname
  4                 ,0, 'Location: '||loc
  5                 ) AS "DATA"
  6    FROM dept,
  7         (SELECT rownum AS row# FROM user_objects WHERE rownum < 4) v
  8   WHERE deptno = 30
  9  /
DATA
--------------------------------------- ---------
Number: 30
Name: SALES
Location: CHICAGO

From Oracle 11g, we can use pivot option

Can one retrieve only rows X to Y from a table?

SELECT * FROM (
   SELECT ename, rownum rn 
            FROM emp WHERE rownum < 101
) WHERE  RN between 91 and 100 ;

Note: the 101 is just one greater than the maximum row of the required rows (means x= 90, y=100, so the inner values is y+1).

SELECT rownum, f1 FROM t1
GROUP BY rownum, f1 HAVING rownum BETWEEN 2 AND 4;

Another solution is to use the MINUS operation. For example, to display rows 5 to 7, construct a query like this:

SELECT *
FROM   tableX
WHERE  rowid in (
   SELECT rowid FROM tableX
    WHERE rownum <= 7
  MINUS
   SELECT rowid FROM tableX
   WHERE rownum < 5);

"this one was faster for me and allowed for sorting before filtering by rownum. The inner query (table A) can be a series of tables joined together with any operation before the filtering by rownum is applied."

SELECT * 
  FROM (SELECT a.*, rownum RN 
	   FROM (SELECT * 
                  FROM t1 ORDER BY key_column) a
         WHERE rownum <=7)
 WHERE rn >=5;

Please note, there is no explicit row order in a relational database. However, this query is quite fun and may even help in the odd situation.

The generic solution to get full information of rows between x and y

---

SELECT * FROM emp WHERE empno in (SELECT empno FROM emp GROUP BY rownum,empno HAVING rownum BETWEEN &x AND &y);

---

"select particular rows from a table : select for rownum = 4, 15 and 17."

select * from ( 
        select rownum myrownum, emp.* from employees emp
        ) mytable 
        where myrownum in (4,15,17);

"selecting row between range of rownum: select for rownum between (12, 20)."

select * from ( 
        select rownum myrownum, emp.* from employees emp
        ) mytable 
        where myrownum between 12 and 20;

"Replace 12 and 20 with &x and &y respectively to assign range dynamically."

select * from ( 
        select rownum myrownum, emp.* from employees emp
        ) mytable 
        where myrownum between &x and &y;

"Combined query to give complete flexibility to pick particular rows and also a given range."

select * from ( 
        select rownum myrownum, emp.* from employees emp
        ) mytable 
        where myrownum between 12 and 17 
        or myrownum in ( 3, 18, 25);

Can one retrieve only the Nth row from a table?

SELECT * FROM t1 a
WHERE  n = (SELECT COUNT(rowid)
              FROM t1 b
             WHERE a.rowid >= b.rowid);

SELECT * FROM (
   SELECT ENAME,ROWNUM RN FROM EMP WHERE ROWNUM < 101 )
 WHERE  RN = 100;

Note: In this first query we select one more than the required row number, then we select the required one. Its far better than using a MINUS operation.

SELECT f1 FROM t1
WHERE  rowid = (
        SELECT rowid FROM t1
        WHERE  rownum <= 10
        MINUS
        SELECT rowid FROM t1
        WHERE  rownum < 10);

SELECT rownum,empno FROM scott.emp a
 GROUP BY rownum,empno HAVING rownum = 4;

Alternatively...

SELECT * FROM emp WHERE rownum=1 AND rowid NOT IN
  (SELECT rowid FROM emp WHERE rownum < 10);

Please note, there is no explicit row order in a relational database. However, this query is quite fun and may even help in the odd situation.

How can one dump/ examine the exact content of a database column?

Table data can be extracted from the database as octal, decimal or hex values:

SELECT DUMP(col1, 10)
FROM tab1
WHERE cond1 = val1;
DUMP(COL1)
-------------------------------------
Typ=96 Len=4: 65,66,67,32

For this example, type=96 is indicating a CHAR column. The last byte in the column is 32, which is the ASCII code for a space. This tells us that this column is blank-padded.

How does one add a column to the middle of a table?

Oracle only allows columns to be added to the end of an existing table. Example:

SQL> CREATE TABLE tab1 ( col1 NUMBER );
Table created.

SQL> ALTER TABLE tab1 ADD (col2 DATE);
Table altered.

SQL> DESC tab1
Name                                      Null?    Type
----------------------------------------- -------- ----------------------------
COL1                                               NUMBER
COL2                                               DATE

Nevertheless, some databases also allow columns to be added to an existing table after a particular column (i.e. in the middle of the table). For example, in MySQL the following syntax is valid:

ALTER TABLE tablename ADD columnname AFTER columnname;

Oracle does not support this syntax. However, it doesn't mean that it cannot be done.

Workarounds:

1. Create a new table and copy the data across.

SQL> RENAME tab1 TO tab1_old;
Table renamed.

SQL> CREATE TABLE tab1 AS SELECT 0 AS col1, col1 AS col2 FROM tab1_old;
Table created.

2. Rename the table and create a view upon it with its former name and with the columns in the order you want.

3. Use the DBMS_REDEFINITION package to change the structure on-line while users are working.

4. Create a view upon the table

SQL> RENAME tab1 TO tab1_original;

Table renamed.

SQL> CREATE VIEW tab1 AS SElECT col2 AS col1, col1 AS col2 FROM tab1_original;

View created.

SQL> desc tab1;
 Name                             Null?    Type
 -------------------------------- -------- ----------------------
 COL1                                      DATE
 COL2                                      NUMBER

How does one code a hierarchical tree-structured query?

The SCOTT/TIGER database schema contains a table EMP with a self-referencing relation (EMPNO and MGR columns). This table is perfect for testing and demonstrating tree-structured queries as the MGR column contains the employee number of the "current" employee's boss.

The LEVEL pseudo-column is an indication of how deep in the tree one is. Oracle can handle queries with a depth of up to 255 levels. Look at this example:

SQL> SELECT     level, empno, ename, mgr
  2    FROM     emp
  3  CONNECT BY PRIOR empno = mgr
  4    START WITH mgr IS NULL
  5  /
     LEVEL      EMPNO ENAME             MGR
---------- ---------- ---------- ----------
         1       7839 KING
         2       7566 JONES            7839
         3       7788 SCOTT            7566
...

One can produce an indented report by using the level number to substring or lpad() a series of spaces, and concatenate that to the string. Look at this example:

SQL> SELECT     LPAD(' ', LEVEL * 2) || ename
  2    FROM     emp
  3  CONNECT BY PRIOR empno = mgr
  4    START WITH mgr IS NULL;
LPAD(,LEVEL*2)||ENAME
------------------------------------------------------
  KING
    JONES
      SCOTT
...

Use the "start with" clause to specify the start of the tree. More than one record can match the starting condition. One disadvantage of having a "connect by prior" clause is that you cannot perform a join to other tables. The "connect by prior" clause is rarely implemented in the other database offerings. Trying to do this programmatically is difficult as one has to do the top level query first, then, for each of the records open a cursor to look for child nodes.

One way of working around this is to use PL/SQL, open the driving cursor with the "connect by prior" statement, and the select matching records from other tables on a row-by-row basis, inserting the results into a temporary table for later retrieval.

NOTE: Tree-structured queries are definitely non-relational (enough to kill Codd and make him roll in his grave). Also, this feature is not often found in other database offerings.

How does one count/sum data values in a column?

Count/sum FIX values:

Use this simple query to count the number of data values in a column:

select my_table_column, count(*)
from   my_table
group  by my_table_column;

A more sophisticated example...

select dept, count(decode(sex,'M',1)) MALE,
             count(decode(sex,'F',1)) FEMALE,
             count(decode(sex,'M',null,'F',null,1)) OTHER,
             count(*) TOTAL
  from my_emp_table
 group by dept;

Count/sum RANGES of data values in a column:

A value x will be between values y and z if GREATEST(x, y) = LEAST(x, z). Look at this example:

select f2,
       sum(decode(greatest(f1,59), least(f1,100), 1, 0)) "Range 60-100",
       sum(decode(greatest(f1,30), least(f1, 59), 1, 0)) "Range 30-59",
       sum(decode(greatest(f1, 0), least(f1, 29), 1, 0)) "Range 00-29"
from   my_table
group  by f2;

For equal size ranges it might be easier to calculate it with DECODE(TRUNC(value/range), 0, rate_0, 1, rate_1, ...). Eg.

select ename "Name", sal "Salary",
       decode( trunc(f2/1000, 0), 0, 0.0,
                                  1, 0.1,
                                  2, 0.2,
                                  3, 0.31) "Tax rate"
from   my_table;

How does one drop/ rename a column in a table?

Drop a column

From Oracle 8i one can DROP a column from a table. Look at this sample script, demonstrating the ALTER TABLE table_name DROP COLUMN column_name; command.

Workarounds for older releases:

SQL> update t1 set column_to_drop = NULL;
SQL> rename t1 to t1_base;
SQL> create view t1 as select <specific columns> from t1_base;

SQL> create table t2 as select <specific columns> from t1;
SQL> drop table t1;
SQL> rename t2 to t1;

Rename a column

From Oracle 9i one can RENAME a column from a table. Look at this example:

ALTER TABLE tablename RENAME COLUMN oldcolumn TO newcolumn;

Workarounds for older releases:

Use a view with correct column names:

rename t1 to t1_base;
create view t1 >column list with new name> as select * from t1_base;

Recreate the table with correct column names:

create table t2 >column list with new name> as select * from t1;
drop table t1;
rename t2 to t1;

Add a column with a new name and drop an old column:

alter table t1 add ( newcolame datatype );  
update t1 set newcolname=oldcolname;
alter table t1 drop column oldcolname;

How does one implement IF-THEN-ELSE logic in a SELECT statement?

One can use the CASE expression or functions like DECODE, NVL, NVL2, NULLIF, COALESCE, etc.

Here is the syntax for the CASE-statement:

CASE exp WHEN comparison_exp1 THEN return_exp1
        [WHEN comparison_exp2 THEN return_exp2
         WHEN comparison_exp3 THEN return_exp3
          ELSE else_exp
        ]
END

And for DECODE:

DECODE( col | exprn, srch1, rslt1 [, srch2, rslt2,...,] [,default] )

How does one prevent Oracle from using an Index?

In certain cases, one may want to disable the use of a specific, or all indexes for a given query. Here are some examples:

Adding an expression to the indexed column

SQL>select count(*) from t where empno+0=1000;
  COUNT(*)
----------
         1

Execution Plan
--------------------------------------------- ----- --------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=3)
   1    0   SORT (AGGREGATE)
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=1 Bytes=3)

Specifying the FULL hint to force full table scan

SQL>select /*+ FULL(t) */ * from t where empno=1000;
     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO GRADE
---------- ---------- --------- ---------- --------- ---------- ---------- ---------- ----------
      1000 Victor     DBA             7839 20-MAY-03      11000          0         10 JUNIOR

Execution Plan
--------------------------------------------- ----- --------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=41)
   1    0   TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=1 Bytes=41)

Specifying NO_INDEX hint

SQL>select /*+ NO_INDEX(T) */ count(*) from t where empno=1000;
  COUNT(*)
----------
         1

Execution Plan
--------------------------------------------- ----- --------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=3)
   1    0   SORT (AGGREGATE)
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=1 Bytes=3)

Using a function over the indexed column

SQL>select count(*) from t where to_number(empno)=1000;
  COUNT(*)
----------
         1

Execution Plan
--------------------------------------------- ----- --------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=3)
   1    0   SORT (AGGREGATE)
   2    1     TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=1 Bytes=3)

How does one select EVERY Nth row from a table?

One can easily select all even, odd, or Nth rows from a table using SQL queries like this:

Method 1: Using a subquery

SELECT *
FROM   emp
WHERE  (ROWID,0) IN (SELECT ROWID, MOD(ROWNUM,4)
                     FROM   emp);

Method 2: Use dynamic views (available from Oracle7.2):

SELECT *
FROM   ( SELECT rownum rn, empno, ename
         FROM emp
       ) temp
WHERE  MOD(temp.ROWNUM,4) = 0;

Method 3: Using GROUP BY and HAVING

SELECT rownum, f1 FROM t1
GROUP BY rownum, f1 HAVING MOD(rownum,n) = 0 OR rownum = 2-n;

Please note, there is no explicit row order in a relational database. However, these queries are quite fun and may even help in the odd situation.

How does one select the LAST N rows from a table?

From Oracle 9i onwards, the RANK() and DENSE_RANK() functions can be used to determine the LAST N or BOTTOM N rows. Examples:

Get the bottom 10 employees based on their salary

SELECT ename, sal 
  FROM ( SELECT ename, sal, RANK() OVER (ORDER BY sal ASC) sal_rank
           FROM emp ) 
 WHERE sal_rank <= 10;

Select the employees getting the lowest 10 salaries

SELECT ename, sal 
  FROM ( SELECT ename, sal, DENSE_RANK() OVER (ORDER BY sal) sal_dense_rank
           FROM emp ) 
 WHERE sal_dense_rank <= 10;

For Oracle 8i and above, one can get the bottom N rows using an inner-query with an ORDER BY clause:

SELECT *
  FROM (SELECT * FROM my_table ORDER BY col_name_1)
 WHERE ROWNUM < 10;

Use this workaround for older (8.0 and prior) releases:

SELECT *
  FROM my_table a
 WHERE 10 >= (SELECT COUNT(DISTINCT maxcol)
                FROM my_table b
               WHERE b.maxcol <= a.maxcol)
 ORDER BY maxcol;

How does one select the TOP N rows from a table?

From Oracle 9i onwards, the RANK() and DENSE_RANK() functions can be used to determine the TOP N rows. Examples:

Get the top 10 employees based on their salary

SELECT ename, sal 
  FROM ( SELECT ename, sal, RANK() OVER (ORDER BY sal DESC) sal_rank
           FROM emp ) 
 WHERE sal_rank <= 10;

Select the employees making the top 10 salaries

SELECT ename, sal 
  FROM ( SELECT ename, sal, DENSE_RANK() OVER (ORDER BY sal DESC) sal_dense_rank
           FROM emp ) 
 WHERE sal_dense_rank <= 10;

For Oracle 8i and above, one can get the Top N rows using an inner-query with an ORDER BY clause:

SELECT *
  FROM (SELECT * FROM my_table ORDER BY col_name_1 DESC)
 WHERE ROWNUM < 10;

Use this workaround for older (8.0 and prior) releases:

SELECT *
  FROM my_table a
 WHERE 10 >= (SELECT COUNT(DISTINCT maxcol)
                FROM my_table b
               WHERE b.maxcol >= a.maxcol)
 ORDER BY maxcol DESC;

How to generate a text graphs (histograms) using SQL?

SELECT d.dname AS "Department",
             LPAD('+', COUNT(*), '+') as "Graph"
  FROM emp e, dept d
 WHERE e.deptno = d.deptno
 GROUP BY d.dname;

Sample output:

Department     Graph
-------------- --------------------------------------------------
ACCOUNTING     +++
RESEARCH       +++++
SALES          ++++++

In the above example, the value returned by COUNT(*) is used to control the number of "*" characters to return for each department. We simply pass COUNT(*) as an argument to the string function LPAD (or RPAD) to return the desired number of *'s.

Map/ concatenate several rows to a column

This FAQ will demonstrate how row values can be concatenated into a single column value (similar to MySQL's GROUP_CONCAT function).

Start by creating this function:

SQL> CREATE OR REPLACE FUNCTION rowconcat(q in VARCHAR2) RETURN VARCHAR2 IS
  2    ret  VARCHAR2(4000);
  3    hold VARCHAR2(4000);
  4    cur  sys_refcursor;
  5  BEGIN
  6    OPEN cur FOR q;
  7    LOOP
  8      FETCH cur INTO hold;
  9      EXIT WHEN cur%NOTFOUND;
 10      IF ret IS NULL THEN
 11        ret := hold;
 12      ELSE
 13        ret := ret || ',' || hold;
 14      END IF;
 15    END LOOP;
 16    RETURN ret;
 17  END;
 18  /
Function created.

This function returns a string result with the concatenated non-NULL values from a SQL statement. It returns NULL if there are no non-NULL values.

Here is an example of how to map several rows to a single concatenated column:

SQL> SELECT rowconcat('SELECT dname FROM dept') AS departments
  2    FROM dual;
DEPARTMENTS
--------------------------------------------------------------------------------
ACCOUNTING,RESEARCH,SALES,OPERATIONS

This example is more interresting, it concatenates a column across several rows based on an aggregation:

SQL> col employees format a50
SQL> SELECT deptno,
  2         rowconcat('SELECT ename FROM emp a WHERE deptno='||deptno) AS Employees
  3    FROM emp
  4   GROUP BY deptno
  5  /
    DEPTNO EMPLOYEES
---------- --------------------------------------------------
        30 ALLEN,WARD,MARTIN,BLAKE,TURNER,JAMES
        20 SMITH,JONES,SCOTT,ADAMS,FORD
        10 CLARK,KING,MILLER

Starting with Oracle11g, you have the LISTAGG function to do such thing.

What is the difference between VARCHAR, VARCHAR2 and CHAR data types?

Both CHAR and VARCHAR2 types are used to store character string values, however, they behave very differently. The VARCHAR type should not be used:

CHAR

CHAR should be used for storing fixed length character strings. String values will be space/blank padded before stored on disk. If this type is used to store variable length strings, it will waste a lot of disk space.

SQL> CREATE TABLE char_test (col1 CHAR(10));
Table created.

SQL> INSERT INTO char_test VALUES ('qwerty');
1 row created.

SQL> SELECT col1, length(col1), dump(col1) "ASCII Dump" FROM char_test;
COL1       LENGTH(COL1) ASCII Dump
---------- ------------ ------------------------------------------------------------
qwerty               10 Typ=96 Len=10: 113,119,101,114,116,121,32,32,32,32

Note: ASCII character 32 is a blank space.

VARCHAR

Currently VARCHAR behaves exactly the same as VARCHAR2. However, this type should not be used as it is reserved for future usage.

SQL> CREATE TABLE varchar_test (col1 VARCHAR2(10));
Table created.

SQL> INSERT INTO varchar_test VALUES ('qwerty');
1 row created.

SQL> SELECT col1, length(col1), dump(col1) "ASCII Dump" FROM varchar_test;
COL1       LENGTH(COL1) ASCII Dump
---------- ------------ ------------------------------------------------------------
qwerty                6 Typ=1 Len=6: 113,119,101,114,116,121

VARCHAR2

VARCHAR2 is used to store variable length character strings. The string value's length will be stored on disk with the value itself.

SQL> CREATE TABLE varchar2_test (col1 VARCHAR2(10));
Table created.

SQL> INSERT INTO varchar2_test VALUES ('qwerty');
1 row created.

SQL> SELECT col1, length(col1), dump(col1) "ASCII Dump" FROM varchar2_test;
COL1       LENGTH(COL1) ASCII Dump
---------- ------------ ------------------------------------------------------------
qwerty                6 Typ=1 Len=6: 113,119,101,114,116,121