MySQL NOT operator support

- Suppot for NOT operator in MySQL, similarly as in PostgreSQL.
- Add test to verify NOT EXISTS query in MySQL

mysql/select_statement_test.go

@@ -132,3 +132,25 @@ FROM db.table1
 LOCK IN SHARE MODE;
 `)
 }
+
+func TestSelect_NOT_EXISTS(t *testing.T) {
+	testutils.AssertStatementSql(t,
+		SELECT(table1ColInt).
+		FROM(table1).
+		WHERE(
+			NOT(EXISTS(
+				SELECT(table2ColInt).
+				FROM(table2).
+				WHERE(
+					table1ColInt.EQ(table2ColInt),
+				),
+		))), `
+SELECT table1.col_int AS "table1.col_int"
+FROM db.table1
+WHERE (NOT (EXISTS (
+          SELECT table2.col_int AS "table2.col_int"
+          FROM db.table2
+          WHERE table1.col_int = table2.col_int
+     )));
+`)
+}