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Friday, July 31, 2015

SQL Server what is Join and what is INNER join

SQL Server what is Join and what is INNER join:


Past Some time when we work in my blog then we got a question related to the sql server the question is

“I am new to SQL Server and want to learn about the JOIN options. What are all of the JOIN options in SQL Server? What is the significance of each of the options? I am a little confused on the differences and syntax, can you provide some examples and explanations? Are JOINs only for SELECT statements?  Check out this tip to learn about SQL Server Joins.”

what is Join and what is INNER join:

Joining tables to obtain the needed data for a query, script or stored procedure is a key concept as you learn about SQL Server development. In a nutshell, joins are typically performed in the FROM clause of a table or view for the SELECT,INSERT...SELECT, SELECT...INTO, UPDATE and DELETE statements. In previous versions of SQL Server, join logic could also have been included in the WHERE clause with = (INNER JOIN), *= (LEFT OUTER JOIN), =* (RIGHT OUTER JOIN), etc. syntax, but the support has been reduced and the best practice in SQL Server is to use the syntax outlined in the examples below.


INNER JOIN - Match rows between the two tables specified in the INNER JOIN statement based on one or more columns having matching data.  Preferably the join is based on referential integrity enforcing the relationship between the tables to ensure data integrity.
Just to add a little commentary to the basic definitions above, in general the INNER JOIN option is considered to be the most common join needed in applications and/or queries.  Although that is the case in some environments, it is really dependent on the database design, referential integrity and data needed for the application.  As such, please take the time to understand the data being requested then select the proper join option.

Although most join logic is based on matching values between the two columns specified, it is possible to also include logic using greater than, less than, not equals, etc.

LEFT OUTER JOIN - Based on the two tables specified in the join clause, all data is returned from the left table.  On the right table, the matching data is returned in addition to NULL values where a record exists in the left table, but not in the right table.
Another item to keep in mind is that the LEFT and RIGHT OUTER JOIN logic is opposite of one another.  

So you can change either the order of the tables in the specific join statement or change the JOIN from left to right or vice versa and get the same results.

RIGHT OUTER JOIN - Based on the two tables specified in the join clause, all data is returned from the right table.  On the left table, the matching data is returned in addition to NULL values where a record exists in the right table but not in the left table.

Self -Join - In this circumstance, the same table is specified twice with two different aliases in order to match the data within the same table.

CROSS JOIN - Based on the two tables specified in the join clause, a Cartesian product is created if a WHERE clause does filter the rows.  The size of the Cartesian product is based on multiplying the number of rows from the left table by the number of rows in the right table.  Please heed caution when using a CROSS JOIN.


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