Implement SQL UNION.

Does not yet work with ORDER BY or LIMIT on the UNION.

sql/parser/sql.y

@@ -1936,7 +1936,7 @@ simple_select:
 | select_clause UNION all_or_distinct select_clause
   {
     $$.val = &Union{
-      Type:  astUnion,
+      Type:  UnionOp,
       Left:  $1.selectStmt(),
       Right: $4.selectStmt(),
       All:   $3.bool(),
@@ -1945,7 +1945,7 @@ simple_select:
 | select_clause INTERSECT all_or_distinct select_clause
   {
     $$.val = &Union{
-      Type:  astIntersect,
+      Type:  IntersectOp,
       Left:  $1.selectStmt(),
       Right: $4.selectStmt(),
       All:   $3.bool(),
@@ -1954,7 +1954,7 @@ simple_select:
 | select_clause EXCEPT all_or_distinct select_clause
   {
     $$.val = &Union{
-      Type:  astExcept,
+      Type:  ExceptOp,
       Left:  $1.selectStmt(),
       Right: $4.selectStmt(),
       All:   $3.bool(),

sql/parser/union.go

@@ -26,18 +26,34 @@ import "fmt"
 
 // Union represents a UNION statement.
 type Union struct {
-	Type        string
+	Type        UnionType
 	Left, Right SelectStatement
 	All         bool
 }
 
+// UnionType represents one of the three set operations in sql.
+type UnionType int
+
 // Union.Type
 const (
-	astUnion     = "UNION"
-	astExcept    = "EXCEPT"
-	astIntersect = "INTERSECT"
+	UnionOp UnionType = iota
+	IntersectOp
+	ExceptOp
 )
 
+var unionTypeName = [...]string{
+	UnionOp:     "UNION",
+	IntersectOp: "INTERSECT",
+	ExceptOp:    "EXCEPT",
+}
+
+func (i UnionType) String() string {
+	if i < 0 || i > UnionType(len(unionTypeName)-1) {
+		return fmt.Sprintf("UnionType(%d)", i)
+	}
+	return unionTypeName[i]
+}
+
 func (node *Union) String() string {
 	all := ""
 	if node.All {

sql/plan.go

@@ -160,6 +160,8 @@ func (p *planner) makePlan(stmt parser.Statement, autoCommit bool) (planNode, *r
 		return p.ShowTables(n)
 	case *parser.Truncate:
 		return p.Truncate(n)
+	case *parser.Union:
+		return p.Union(n, autoCommit)
 	case *parser.Update:
 		return p.Update(n, autoCommit)
 	case *parser.Values:
@@ -321,6 +323,7 @@ var _ planNode = &scanNode{}
 var _ planNode = &sortNode{}
 var _ planNode = &valuesNode{}
 var _ planNode = &selectNode{}
+var _ planNode = &unionNode{}
 var _ planNode = &emptyNode{}
 var _ planNode = &explainDebugNode{}
 var _ planNode = &explainTraceNode{}

sql/testdata/union

@@ -0,0 +1,134 @@
+query I rowsort
+VALUES (1), (1), (1), (2), (2) UNION VALUES (1), (3), (1)
+----
+1
+2
+3
+
+query I rowsort
+VALUES (1), (1), (1), (2), (2) UNION ALL VALUES (1), (3), (1)
+----
+1
+1
+1
+1
+1
+2
+2
+3
+
+query I rowsort
+VALUES (1), (1), (1), (2), (2) INTERSECT VALUES (1), (3), (1)
+----
+1
+
+query I rowsort
+VALUES (1), (1), (1), (2), (2) INTERSECT ALL VALUES (1), (3), (1)
+----
+1
+1
+
+query I rowsort
+VALUES (1), (1), (1), (2), (2) EXCEPT VALUES (1), (3), (1)
+----
+2
+
+query I rowsort
+VALUES (1), (1), (1), (2), (2) EXCEPT ALL VALUES (1), (3), (1)
+----
+1
+2
+2
+
+query II rowsort
+VALUES (1, 2), (1, 1), (1, 2), (2, 1), (2, 1) UNION VALUES (1, 3), (3, 4), (1, 1)
+----
+1 1
+1 2
+1 3
+2 1
+3 4
+
+statement ok
+CREATE TABLE uniontest (
+  k INT,
+  v INT
+)
+
+statement OK
+INSERT INTO uniontest VALUES
+(1, 1),
+(1, 1),
+(1, 1),
+(1, 2),
+(1, 2),
+(2, 1),
+(2, 3),
+(2, 1)
+
+query I rowsort
+SELECT v FROM uniontest WHERE k = 1 UNION SELECT v FROM uniontest WHERE k = 2
+----
+1
+2
+3
+
+query I rowsort
+SELECT v FROM uniontest WHERE k = 1 UNION ALL SELECT v FROM uniontest WHERE k = 2
+----
+1
+1
+1
+1
+1
+2
+2
+3
+
+query I rowsort
+SELECT v FROM uniontest WHERE k = 1 INTERSECT SELECT v FROM uniontest WHERE k = 2
+----
+1
+
+query I rowsort
+SELECT v FROM uniontest WHERE k = 1 INTERSECT ALL SELECT v FROM uniontest WHERE k = 2
+----
+1
+1
+
+query I rowsort
+SELECT v FROM uniontest WHERE k = 1 EXCEPT SELECT v FROM uniontest WHERE k = 2
+----
+2
+
+query I rowsort
+SELECT v FROM uniontest WHERE k = 1 EXCEPT ALL SELECT v FROM uniontest WHERE k = 2
+----
+1
+2
+2
+
+query ITT rowsort
+EXPLAIN SELECT v FROM uniontest UNION SELECT k FROM uniontest
+----
+0 union -
+1 scan uniontest@primary
+1 scan uniontest@primary
+
+query error each UNION query must have the same number of columns: 2 vs 1
+SELECT 1, 2 UNION SELECT 3
+
+query error each INTERSECT query must have the same number of columns: 2 vs 1
+SELECT 1, 2 INTERSECT SELECT 3
+
+query error each EXCEPT query must have the same number of columns: 2 vs 1
+SELECT 1, 2 EXCEPT SELECT 3
+
+query error UNION types int and string cannot be matched
+SELECT 1 UNION SELECT '3'
+
+query error INTERSECT types int and string cannot be matched
+SELECT 1 INTERSECT SELECT '3'
+
+query error EXCEPT types int and string cannot be matched
+SELECT 1 EXCEPT SELECT '3'