identifier = expr
The equal sign is used as both an assignment operator in certain contexts, and as a comparison operator. When used as assignment operator, the value on the right is assigned to the variable (or column, in some contexts) on the left.
Since its use can be ambiguous, unlike the := assignment operator, the
= assignment operator cannot be used in all contexts, and is only valid as part of a SET statement, or the SET clause of an UPDATE statement
UPDATE table_name SET x = 2 WHERE x > 100;
SET @x = 1, @y := 2;
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