Syntax:
=

This operator is used to perform value assignments in two cases,
described in the next two paragraphs.

Within a SET statement, = is treated as an assignment operator that
causes the user variable on the left hand side of the operator to take
on the value to its right. (In other words, when used in a SET
statement, = is treated identically to :=.) The value on the right hand
side may be a literal value, another variable storing a value, or any
legal expression that yields a scalar value, including the result of a
query (provided that this value is a scalar value). You can perform
multiple assignments in the same SET statement.

In the SET clause of an UPDATE statement, = also acts as an assignment
operator; in this case, however, it causes the column named on the left
hand side of the operator to assume the value given to the right,
provided any WHERE conditions that are part of the UPDATE are met. You
can make multiple assignments in the same SET clause of an UPDATE
statement.

In any other context, = is treated as a comparison operator.

URL: http://dev.mysql.com/doc/refman/5.7/en/assignment-operators.html

