Syntax:
:=

Assignment operator. Causes the user variable on the left hand side of
the operator to take on the value to its right. 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. You can perform
multiple assignments in the same statement-

Unlike =, the := operator is never interpreted as a comparison
operator. This means you can use := in any valid SQL statement (not
just in SET statements) to assign a value to a variable.

URL: http://dev.mysql.com/doc/refman/5.6/en/assignment-operators.html

