Good back-to-basics question, thanks

The final effect is the same, i.e. the variable @STR is assigned the value of 1.

There are numerous minor differences between the actions.

Read about SET and SELECT.

the main difference is that the SET can be used to allocate a value to one variable at a time, while a SELECT can allocate multiple at the same time, e.g.

DECLARE @Var1 INT, @Var2 VARCHAR(25)

SET @Var1 = 5;

SET @Var2 = 'Hello ';

--OR--

SELECT @Var1 = 5, @Var2 = 'Hello '

With the advent of SQL2008, it is now possible to assign a value to a variable when declaring it (just as is done in coding in e.g. C# or vb, e.g.

DECLARE @Var1 INT = 5, @Var2 VARCHAR(25) = 'Hello '

SELECT @Var1 *= @Var1, -- will render as 25

@Var2 += SUSER_SNAME()

SELECT @Var1, @Var2