SET vs SELECT when assigning variables

SET is the ANSI standard for variable assignment, SELECT is not.


SET can only assign one variable at a time, SELECT can make multiple assignments at once.

If assigning from a query, SET can only assign a scalar value. If the query returns multiple values/rows then SET will raise an error. SELECT will assign one of the values to the variable and hide the fact that multiple values were returned (so you'd likely never know why something was going wrong elsewhere - have fun troubleshooting that one)

When assigning from a query if there is no value returned then SET will assign NULL, where SELECT will not make the assignment at all (so the variable will not be changed from it's previous value)

As far as speed differences - there are no direct differences between SET and SELECT. However SELECT's ability to make multiple assignments in one shot does give it a slight speed advantage over SET.

SQL Server Join Hints

JOIN hints can be used in a query to specify the type of JOIN the Query Optimizer is to use for the execution plan. The JOIN options are:

• Loop
• Merge
• Hash

The syntax for a JOIN hint is (using an INNER JOIN as an example):

FROM table_one INNER [LOOP | MERGE | JOIN] JOIN table_two

Here's an example:

FROM header_table INNER LOOP JOIN detail_table

As you can see, the JOIN hint is between the type of JOIN (in this example, INNER) and the JOIN keyword. Only one JOIN hint can be used per JOIN in a query. In addition, JOIN hints can only be used when the ANSI JOIN syntax is used, not the older Microsoft JOIN syntax.

The syntax above is not the only option to add a JOIN hint to a query. In addition, you can also use OPTION clause. Using the OPTION clause specifies that the hint be used throughout all of the query. While multiple hints can be added to the OPTION clause, each query hint can be used only once. In addition, only one OPTION clause can be used per query.

Here's an example of using the OPTION clause:

OPTION (INNER) or OPTION (MERGE) or OPTION (HASH)

The Query Optimizer always tries to identify the fastest way to JOIN tables. The fastest JOIN method is the Loop JOIN, followed by the Merge and the Hash JOIN. While the Query Optimizer always tries to perform a Loop JOIN if possible, it is not always possible, and one of the other two types may have to be used.