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;

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;

2. 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;

3. 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;

4. 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

5. 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;

6. 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;

7. 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.

8. 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 '.');

9. 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;

10. 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;

CGI or Common Gateway Interface:

CGI or Common Gateway Interface:

To understand the concept of CGI, let's see what happens when we click a hyperlink to browse a particular web page or URL.

    Your browser contacts the HTTP web server and demand for the URL ie. filename.

    Web Server will parse the URL and will look for the filename. If it finds requested file then web server sends that file back to the browser otherwise sends an error message indicating that you have requested a wrong file.

    Web browser takes response from web server and displays either the received file or error message based on the received response.

However, it is possible to set up the HTTP server in such a way that whenever a file in a certain directory is requested, that file is not sent back; instead it is executed as a program, and produced output from the program is sent back to your browser to display.

The Common Gateway Interface (CGI) is a standard protocol for enabling applications (called CGI programs or CGI scripts) to interact with Web servers and with clients. These CGI programs can be written in Python, PERL, Shell, C or C++ etc.