Tuesday 5 August 2014

Database Administration Queries

Database administration queries

  1. Database version information
Returns the Oracle database version.
SELECT * FROM v$version;
  1. Database default information
Some system default information.
SELECT username,
       profile,
       default_tablespace,
       temporary_tablespace
  FROM dba_users;
  1. Database Character Set information
Display the character set information of database.
SELECT * FROM nls_database_parameters;
  1. Get Oracle version
SELECT VALUE
  FROM v$system_parameter
 WHERE name = 'compatible';
  1. 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;
  1. Resizing Tablespace without adding datafile
Yet another DDL query to resize table space.
ALTER DATABASE DATAFILE '/work/oradata/STARTST/STAR02D.dbf' resize 2000M;
  1. 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;
  1. 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;
  1. 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;
  1. 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;
  1. 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;
  1. 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';
  1. Last SQL fired by the User on Database
This query will display last SQL query fired by each user in this database. Notice how this query display last SQL per each session.
SELECT S.USERNAME || '(' || s.sid || ')-' || s.osuser UNAME,
         s.program || '-' || s.terminal || '(' || s.machine || ')' PROG,
         s.sid || '/' || s.serial# sid,
         s.status "Status",
         p.spid,
         sql_text sqltext
    FROM v$sqltext_with_newlines t, V$SESSION s, v$process p
   WHERE     t.address = s.sql_address
         AND p.addr = s.paddr(+)
         AND t.hash_value = s.sql_hash_value
ORDER BY s.sid, t.piece;

Monday 14 July 2014

Data dictionary queries

Data dictionary queries


Data dictionary queries

1.      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';

2.      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';

3.      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;

4.      Getting current schema

Yet another query to get current schema name.
SELECT SYS_CONTEXT ('userenv', 'current_schema') FROM DUAL;

5.      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;

Monday 7 July 2014

10 Most Important Date / Time related queries

10 Most Important Date / Time related queries


10 Most Important Date / Time related queries

  1. Get the first day of the month
Quickly returns the first day of current month. Instead of current month you want to find first day of month where a date falls, replace SYSDATE with any date column/value.
SELECT TRUNC (SYSDATE, 'MONTH') "First day of current month"
    FROM DUAL;
  1. Get the last day of the month
This query is similar to above but returns last day of current month. One thing worth noting is that it automatically takes care of leap year. So if you have 29 days in Feb, it will return 29/2. Also similar to above query replace SYSDATE with any other date column/value to find last day of that particular month.
SELECT TRUNC (LAST_DAY (SYSDATE)) "Last day of current month"
    FROM DUAL;
  1. Get the first day of the Year
First day of year is always 1-Jan. This query can be use in stored procedure where you quickly want first day of year for some calculation.
SELECT TRUNC (SYSDATE, 'YEAR') "Year First Day" FROM DUAL;
  1. Get the last day of the year
Similar to above query. Instead of first day this query returns last day of current year.
SELECT ADD_MONTHS (TRUNC (SYSDATE, 'YEAR'), 12) - 1 "Year Last Day" FROM DUAL
  1. Get number of days in current month
Now this is useful. This query returns number of days in current month. You can change SYSDATE with any date/value to know number of days in that month.
SELECT CAST (TO_CHAR (LAST_DAY (SYSDATE), 'dd') AS INT) number_of_days
  FROM DUAL;
  1. Get number of days left in current month
Below query calculates number of days left in current month.
SELECT SYSDATE,
       LAST_DAY (SYSDATE) "Last",
       LAST_DAY (SYSDATE) - SYSDATE "Days left"
  FROM DUAL;
  1. Get number of days between two dates
Use this query to get difference between two dates in number of days.
SELECT ROUND ( (MONTHS_BETWEEN ('01-Feb-2014', '01-Mar-2012') * 30), 0)
          num_of_days
  FROM DUAL;

OR

SELECT TRUNC(sysdate) - TRUNC(e.hire_date) FROM employees;
Use second query if you need to find number of days since some specific date. In this example number of days since any employee is hired.
  1. 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 '.');
  1. 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;
  1. 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;