For a missing value, MySQL sets the column to the implicit default for its data type. Implicit defaults were described in Section 3.2.3, “Specifying Column Default Values.†...

Assign a value to a variable in a "SELECT" statement:SELECT @fruit1, @fruit2, @fruit3, @fruit4:='mango';You must use ":=" if you are assigning a value to a variable in any other type of statement ...