*: add Not flag for ExistsSubqueryExpr (#391)

ast/expressions.go

@@ -443,6 +443,8 @@ type ExistsSubqueryExpr struct {
 	exprNode
 	// Sel is the subquery, may be rewritten to other type of expression.
 	Sel ExprNode
+	// Not is true, the expression is "not exists".
+	Not bool
 }
 
 // Format the ExprNode into a Writer.

parser.y

@@ -2568,7 +2568,13 @@ Expression:
 	}
 |	"NOT" Expression %prec not
 	{
-		$$ = &ast.UnaryOperationExpr{Op: opcode.Not, V: $2}
+		expr, ok := $2.(*ast.ExistsSubqueryExpr)
+		if ok {
+			expr.Not = true
+			$$ = $2
+		} else {
+			$$ = &ast.UnaryOperationExpr{Op: opcode.Not, V: $2}
+		}
 	}
 |	BoolPri IsOrNotOp trueKwd %prec is
 	{

parser_test.go

@@ -2704,3 +2704,20 @@ func (s *testParserSuite) TestFieldText(c *C) {
 		c.Assert(traceStmt.Stmt.Text(), Equals, "select a from t")
 	}
 }
+
+func (s *testParserSuite) TestNotExistsSubquery(c *C) {
+	table := []testCase{
+		{`select * from t1 where not exists (select * from t2 where t1.a = t2.a)`, true},
+	}
+
+ 	parser := New()
+	for _, tt := range table {
+		stmt, _, err := parser.Parse(tt.src, "", "")
+		c.Assert(err, IsNil)
+
+ 		sel := stmt[0].(*ast.SelectStmt)
+		exists, ok := sel.Where.(*ast.ExistsSubqueryExpr)
+		c.Assert(ok, IsTrue)
+		c.Assert(exists.Not, Equals, tt.ok)
+	}
+}