From dfa09452ca06ce03fe82393eedb8c8bbe076782d Mon Sep 17 00:00:00 2001
From: Miguel Molina <miguel@erizocosmi.co>
Date: Tue, 4 Jun 2019 11:55:27 +0200
Subject: [PATCH] sql/expression: allow null literals in case branches

Signed-off-by: Miguel Molina <miguel@erizocosmi.co>
---
 engine_test.go                   |  4 ++++
 sql/analyzer/validation_rules.go |  6 +++---
 sql/expression/case.go           | 11 +++++++++--
 sql/expression/case_test.go      | 19 ++++++++++++++++++-
 4 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/engine_test.go b/engine_test.go
index 1b0d39cdc..dfda34bb2 100644
--- a/engine_test.go
+++ b/engine_test.go
@@ -1230,6 +1230,10 @@ var queries = []struct {
 		`SELECT SUBSTR(SUBSTRING('0123456789ABCDEF', 1, 10), -4)`,
 		[]sql.Row{{"6789"}},
 	},
+	{
+		`SELECT CASE i WHEN 1 THEN i ELSE NULL END FROM mytable`,
+		[]sql.Row{{int64(1)}, {nil}, {nil}},
+	},
 }
 
 func TestQueries(t *testing.T) {
diff --git a/sql/analyzer/validation_rules.go b/sql/analyzer/validation_rules.go
index 06d1f13b6..e7448d9e2 100644
--- a/sql/analyzer/validation_rules.go
+++ b/sql/analyzer/validation_rules.go
@@ -3,11 +3,11 @@ package analyzer
 import (
 	"strings"
 
-	"gopkg.in/src-d/go-errors.v1"
 	"github.com/src-d/go-mysql-server/sql"
 	"github.com/src-d/go-mysql-server/sql/expression"
 	"github.com/src-d/go-mysql-server/sql/expression/function"
 	"github.com/src-d/go-mysql-server/sql/plan"
+	"gopkg.in/src-d/go-errors.v1"
 )
 
 const (
@@ -242,14 +242,14 @@ func validateCaseResultTypes(ctx *sql.Context, a *Analyzer, n sql.Node) (sql.Nod
 		case *expression.Case:
 			typ := e.Type()
 			for _, b := range e.Branches {
-				if b.Value.Type() != typ {
+				if b.Value.Type() != typ && b.Value.Type() != sql.Null {
 					err = ErrCaseResultType.New(typ, b.Value, b.Value.Type(), e)
 					return false
 				}
 			}
 
 			if e.Else != nil {
-				if e.Else.Type() != typ {
+				if e.Else.Type() != typ && e.Else.Type() != sql.Null {
 					err = ErrCaseResultType.New(typ, e.Else, e.Else.Type(), e)
 					return false
 				}
diff --git a/sql/expression/case.go b/sql/expression/case.go
index 388f3a7ee..28eef6d03 100644
--- a/sql/expression/case.go
+++ b/sql/expression/case.go
@@ -28,9 +28,16 @@ func NewCase(expr sql.Expression, branches []CaseBranch, elseExpr sql.Expression
 // Type implements the sql.Expression interface.
 func (c *Case) Type() sql.Type {
 	for _, b := range c.Branches {
-		return b.Value.Type()
+		if b.Value.Type() != sql.Null {
+			return b.Value.Type()
+		}
+	}
+
+	if c.Else.Type() != sql.Null {
+		return c.Else.Type()
 	}
-	return c.Else.Type()
+
+	return sql.Null
 }
 
 // IsNullable implements the sql.Expression interface.
diff --git a/sql/expression/case_test.go b/sql/expression/case_test.go
index 6319db2d7..80a24aa47 100644
--- a/sql/expression/case_test.go
+++ b/sql/expression/case_test.go
@@ -3,8 +3,8 @@ package expression
 import (
 	"testing"
 
-	"github.com/stretchr/testify/require"
 	"github.com/src-d/go-mysql-server/sql"
+	"github.com/stretchr/testify/require"
 )
 
 func TestCase(t *testing.T) {
@@ -127,3 +127,20 @@ func TestCase(t *testing.T) {
 		})
 	}
 }
+
+func TestCaseNullBranch(t *testing.T) {
+	require := require.New(t)
+	f := NewCase(
+		NewGetField(0, sql.Int64, "x", false),
+		[]CaseBranch{
+			{
+				Cond:  NewLiteral(int64(1), sql.Int64),
+				Value: NewLiteral(nil, sql.Null),
+			},
+		},
+		nil,
+	)
+	result, err := f.Eval(sql.NewEmptyContext(), sql.Row{int64(1)})
+	require.NoError(err)
+	require.Nil(result)
+}