Genpact Oracle SQL Queries In Written Test Interview

Genpact Frequently Asked Oracle SQL Queries In Written Test Interview Questions

Display each months start and end date upto last month of the year

This clever query displays start date and end date of each month in current year. You might want to use this for certain types of calculations.

SELECT ADD_MONTHS (TRUNC (SYSDATE, 'MONTH'), i) start_date,
       TRUNC (LAST_DAY (ADD_MONTHS (SYSDATE, i))) end_date
  FROM XMLTABLE (
          'for $i in 0 to xs:int(D) return $i'
          PASSING XMLELEMENT (
                     d,
                     FLOOR (
                        MONTHS_BETWEEN (
                           ADD_MONTHS (TRUNC (SYSDATE, 'YEAR') - 1, 12),
                           SYSDATE)))
          COLUMNS i INTEGER PATH '.');

Get number of seconds passed since today (since 00:00 hr)

SELECT (SYSDATE - TRUNC (SYSDATE)) * 24 * 60 * 60 num_of_sec_since_morning
  FROM DUAL;

Get number of seconds left today (till 23:59:59 hr)

SELECT (TRUNC (SYSDATE+1) - SYSDATE) * 24 * 60 * 60 num_of_sec_left
  FROM DUAL;

Genpact Frequently Asked Oracle SQL Queries In Written Test Interview Questions

Check if a table exists in the current database schema

A simple query that can be used to check if a table exists before you create it. This way you can make your create table script rerunnable. Just replace table_name with actual table you want to check. This query will check if table exists for current user (from where the query is executed).

SELECT table_name
  FROM user_tables
 WHERE table_name = 'TABLE_NAME';

Check if a column exists in a table

Simple query to check if a particular column exists in table. Useful when you tries to add new column in table using ALTER TABLE statement, you might wanna check if column already exists before adding one.

SELECT column_name AS FOUND
  FROM user_tab_cols
 WHERE table_name = 'TABLE_NAME' AND column_name = 'COLUMN_NAME';

Showing the table structure

This query gives you the DDL statement for any table. Notice we have pass ‘TABLE’ as first parameter. This query can be generalized to get DDL statement of any database object. For example to get DDL for a view just replace first argument with ‘VIEW’ and second with your view name and so.

SELECT DBMS_METADATA.get_ddl ('TABLE', 'TABLE_NAME', 'USER_NAME') FROM DUAL;

Getting current schema

Yet another query to get current schema name.

SELECT SYS_CONTEXT ('userenv', 'current_schema') FROM DUAL;

Changing current schema

Yet another query to change the current schema. Useful when your script is expected to run under certain user but is actually executed by other user. It is always safe to set the current user to what your script expects.

ALTER SESSION SET CURRENT_SCHEMA = new_schema;

Database version information

Returns the Oracle database version.

SELECT * FROM v$version;

Database default information

Some system default information.

SELECT username,
       profile,
       default_tablespace,
       temporary_tablespace
  FROM dba_users;

Database Character Set information

Display the character set information of database.

SELECT * FROM nls_database_parameters;

Get Oracle version

SELECT VALUE
  FROM v$system_parameter
 WHERE name = 'compatible';

Store data case sensitive but to index it case insensitive

Now this ones tricky. Sometime you might querying database on some value independent of case. In your query you might do UPPER(..) = UPPER(..) on both sides to make it case insensitive. Now in such cases, you might want to make your index case insensitive so that they don’t occupy more space. Feel free to experiment with this one.

CREATE TABLE tab (col1 VARCHAR2 (10));

CREATE INDEX idx1
   ON tab (UPPER (col1));

ANALYZE TABLE a COMPUTE STATISTICS;

Resizing Tablespace without adding datafile

Yet another DDL query to resize table space.

ALTER DATABASE DATAFILE '/work/oradata/STARTST/STAR02D.dbf' resize 2000M;

Checking autoextend on/off for Tablespaces

Query to check if autoextend is on or off for a given tablespace.

SELECT SUBSTR (file_name, 1, 50), AUTOEXTENSIBLE FROM dba_data_files;

(OR)

SELECT tablespace_name, AUTOEXTENSIBLE FROM dba_data_files;

Adding datafile to a tablespace

Query to add datafile in a tablespace.

ALTER TABLESPACE data01 ADD DATAFILE '/work/oradata/STARTST/data01.dbf'
    SIZE 1000M AUTOEXTEND OFF;

Increasing datafile size

Yet another query to increase the datafile size of a given datafile.

ALTER DATABASE DATAFILE '/u01/app/Test_data_01.dbf' RESIZE 2G;

Find the Actual size of a Database

Gives the actual database size in GB.

SELECT SUM (bytes) / 1024 / 1024 / 1024 AS GB FROM dba_data_files;

Find the size occupied by Data in a Database or Database usage details

Gives the size occupied by data in this database.

SELECT SUM (bytes) / 1024 / 1024 / 1024 AS GB FROM dba_segments;

Find the size of the SCHEMA/USER

Give the size of user in MBs.

SELECT SUM (bytes / 1024 / 1024) "size"
  FROM dba_segments
 WHERE owner = '&owner';