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What are the differences between the SET and SELECT statements when assigning variables in T-SQL?
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SET is used to assign a value to a variable and SELECT is used to assign a value or to select value from a variable/table/view etc.
When you use select * you're make it impossible to profile, therefore you're not writing clear & straightforward code and you are going against the spirit of the quote. select * is an anti-pattern. So selecting columns is not a premature optimization.
To assign a value to a variable, use the SET statement. This is the preferred method of assigning a value to a variable. A variable can also have a value assigned by being referenced in the select list of a SELECT statement.
Whenever you are assigning a query returned value to a variable, SET will accept and assign a scalar (single) value from a query. While SELECT could accept multiple returned values.
Quote, which summarizes from this article:
- 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 its 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.
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