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by Dinesh Thakur Category: SQL Commands

A self-join is a query in which a table is joined (compared) to itself. Self-joins are used to compare values in a column with other values in the same column in the same table. The SQL self-join can be done by using table aliases to cheat one table like a different table and then join them together.

 
by Dinesh Thakur Category: SQL Commands

In SQL inner joins are also called simple joins or equijoin. We are now ready to present a SELECT statement with what is called an inner join:

 
by Dinesh Thakur Category: SQL Commands

The result of a left outer join (or simply left join) for table A and B always contains records of the "left" table (A), even if the join-condition does not find any matching record in the "right" table (B). This means that if the ON clause matches 0 (zero) records in B, the join will still return a row in the result-but with NULL in each column from B.

 
by Dinesh Thakur Category: SQL Commands

A cross join, or product provides the foundation upon which all types of inner joins operate. A cross join returns the Cartesian product of the sets of records from the two joined tables. Thus, it equates to an inner join where the join-condition always evaluates to true or join-condition is absent in statement. This command is introduced for Oracle9i. If A and B are two sets, then the cross join is written as A x B.

 
by Dinesh Thakur Category: SQL Commands

An Index can be created on a single column or a combination of columns in a database table. A table index is a database structure that arranges the values of one or more columns in a database table in specific order. The table index has pointers to the values stored in specified column or combination of columns of the table. These pointers are ordered depending on the sort order specified in the index.



 
by Dinesh Thakur Category: SQL Commands

A right outer join (or right join) closely resembles a left outer join, except with the treatment of the tables reversed. Every row from the "right" table (B) will appear in the Joined table at least once. If no matching row from the "left" table (A) exists, NULL will appear in columns from A for those records that have no match in A.

 
by Dinesh Thakur Category: SQL Commands

The SQL DROP INDEX statement is the SQL command that removes an entire SQL index. You may drop an index permanently when it is no longer useful or temporarily. If the index is harming or not helping performance it could be dropped.

 
by Dinesh Thakur Category: SQL Commands

When an index is created by using multiple columns then it is known as Composite index .

• Create an index on the NAME AND SALARY columns of the employees table.

 
by Dinesh Thakur Category: SQL Functions

The simplest of the date/time functions is one that returns the current date and time. In Microsoft SQL Server, the function is named GETDATE. This function has no arguments. It merely returns the current date and time. For example:

 
by Dinesh Thakur Category: SQL Functions

The NVL and NVL2 functions allow you to test an expression to see whether it is NULL. If an expression is NULL, you can return an alternate, non-NULL value, to use in its place. Since any of the expressions in a DECODE statement can be NULL, the NVL and NVL2 functions are actually specialized versions of DECODE. The following example uses NVL2 to produce the same results as the DECODE:

 

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About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.SC (Computer Science), MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular Computer Notes blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to contact us.



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