In the stored procedure, select and set assign values to variables.

SQL Server assigns values to Defined variables in two ways: Set and select. The difference between the two methods has been described in detail in the SQL Server online series, but we do not pay attention to the fact that there are many differences between the two methods.

Set instead of select is recommended for SQL Server. When the expression returns a value and assigns a value to a variable, the set method is recommended.

The following table lists the differences between set and select.

 

	Set	Select
Assign values to multiple variables at the same time	Not Supported	Supported
When the expression returns multiple values	Error	Assign the last returned value to the variable.
The expression does not return a value.	The variable is assigned a null value.	Keep original variable value

The following is an example to illustrate the problem:

Create Table chinadba_a (
Userid int,
ADDR varchar (128)
)
Go
Insert into chinadba_a (userid, ADDR) values (1, 'addr1 ')
Insert into chinadba_a (userid, ADDR) values (2, 'addr2 ')
Insert into chinadba_a (userid, ADDR) values (3, 'addr3 ')
Go

When the expression returns multiple values, use set to assign values.

Declare @ ADDR varchar (128)
Set @ ADDR = (select ADDR from chinadba_a)

/*
Error message: SERVER: Message 512, level 16, status 1, Row 2. The subquery returns more than one value. When the subquery follows in = ,! =, <, <=,>,> =, Or use a subquery as an expression.
*/
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When the expression returns multiple values, use select to assign values.

Declare @ ADDR varchar (128)
Select @ ADDR = ADDR from chinadba_a
Print @ ADDR -- Value of the last ADDR column in The result set
-- Result: addr3
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If the expression does not return a value, use set to assign a value.

Declare @ ADDR varchar (128)
Set @ ADDR = 'Initiator'
Set @ ADDR = (select ADDR from chinadba_a where userid = 4)
Print @ ADDR -- Null Value
Go

If the expression does not return a value, use select to assign a value.

Declare @ ADDR varchar (128)
Set @ ADDR = 'Initiator'
Select @ ADDR = ADDR from chinadba_a where userid = 4
Print @ ADDR -- keep Original Value
Go

Note that select can also assign the value of the scalar quantum query to the variable. If the scalar query does not return a value, the variable is set to null. In this case, the concept of scalar quantum query, which is identical to the use of set assignment, should be unfamiliar to everyone. For example, we can illustrate

Declare @ ADDR varchar (128)
Set @ ADDR = 'Initiator'
-- Select ADDR from chinadba_a where userid = 4: Scalar query statement
Select @ ADDR = (select ADDR from chinadba_a where userid = 4)
Print @ ADDR -- Null Value
Go

In the stored procedure, select and set assign values to variables.