The difference between select and SET on variable assignment in SQL Server stored procedures _mssql

SQL Server recommends assigning a variable using SET instead of SELECT.
The SET method is recommended when an expression returns a value and assigns a variable.
The following table lists the differences between SET and SELECT. Please pay special attention to the red section.

	Set	Select
Assign values to multiple variables at the same time	does not support	Support
When an expression returns multiple values	Error	Assigns the last value returned to a variable
Expression does not return a value	variable is assigned null value	Variable keeps the original value

Here's a concrete example to illustrate the problem:
CREATE TABLE Chinadba1 (
UserID int,
Addr varchar (128)
)
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Insert into CHINADBA1 (USERID,ADDR) VALUES (1, ' ADDR1 ')
Insert into CHINADBA1 (USERID,ADDR) VALUES (2, ' ADDR2 ')
Insert into CHINADBA1 (USERID,ADDR) VALUES (3, ' ADDR3 ')
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When an expression returns multiple values, the SET assignment is used
DECLARE @addr varchar (128)
Set @addr = (select addr from Chinadba1)
/*
--The error message is
Server: Message 512, Level 16, State 1, line 2
The subquery returns more than one value. This is not allowed when the subquery follows =,!=, <, <=, >, >=, or the subquery is used as an expression.
*/
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When an expression returns multiple values, declare @addr varchar (128) using the SELECT assignment.
Select @addr = addr from Chinadba1
Print @addr--the value of the last addr column in the result set
--Results: ADDR3
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When an expression does not return a value, the SET assignment declare @addr varchar (128)
Set @addr = ' initial value '
Set @addr = (select addr from chinadba1 where UserID = 4)
Print @addr--null value
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When the expression does not return a value, use SELECT to assign declare @addr varchar (128)
Set @addr = ' initial value '
Select @addr = addr from chinadba1 where UserID = 4
Print @addr--Keep the original value
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It should be noted that SELECT can also assign the value of a scalar quantum query to a variable, and if the scalar query does not return a value, the variable is set to a null value.
This is exactly the same as using the SET assignment.
The concept of the standard quantum query should all be unfamiliar, for example, we can explain
DECLARE @addr varchar (128)
Set @addr = ' initial value '
--select addr from chinadba1 where UserID = 4 for standard Quantum query statements
Select @addr = (select addr from chinadba1 where UserID = 4)
Print @addr--null value
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