compiler: support UNION query

This commit adds support for UNION query in the compiler. It adds test cases
for UNION in a SELECT and in a recursive CTE.

It fixes sqlc-dev#568, sqlc-dev#723 and sqlc-dev#778.

internal/compiler/output_columns.go

@@ -37,6 +37,11 @@ func outputColumns(qc *QueryCatalog, node ast.Node) ([]*Column, error) {
 		targets = n.ReturningList
 	case *ast.SelectStmt:
 		targets = n.TargetList
+		// For UNION queries, targets is empty and we need to look for the
+		// columns in Largs.
+		if len(targets.Items) == 0 && n.Larg != nil {
+			return outputColumns(qc, n.Larg)
+		}
 	case *ast.TruncateStmt:
 		targets = &ast.List{}
 	case *ast.UpdateStmt:
@@ -199,6 +204,7 @@ func outputColumns(qc *QueryCatalog, node ast.Node) ([]*Column, error) {
 
 		}
 	}
+
 	return cols, nil
 }
 

internal/endtoend/testdata/cte_recursive/query.sql

@@ -0,0 +1,11 @@
+CREATE TABLE bar (id INT NOT NULL, parent_id INT);
+
+-- name: CTERecursive :many
+WITH RECURSIVE cte AS (
+        SELECT b.* FROM bar AS b
+        WHERE b.id = $1
+    UNION ALL
+        SELECT b.*
+        FROM bar AS b, cte AS c
+        WHERE b.parent_id = c.id
+) SELECT * FROM cte;

internal/endtoend/testdata/cte_recursive/sqlc.json

@@ -0,0 +1,11 @@
+{
+  "version": "1",
+  "packages": [
+    {
+      "path": "go",
+      "name": "querytest",
+      "schema": "query.sql",
+      "queries": "query.sql"
+    }
+  ]
+}

internal/endtoend/testdata/select_union/query.sql

@@ -0,0 +1,6 @@
+CREATE TABLE foo (a text, b text);
+
+-- name: SelectUnion :many
+SELECT * FROM foo
+UNION
+SELECT * FROM foo;

internal/endtoend/testdata/select_union/sqlc.json

@@ -0,0 +1,11 @@
+{
+  "version": "1",
+  "packages": [
+    {
+      "path": "go",
+      "name": "querytest",
+      "schema": "query.sql",
+      "queries": "query.sql"
+    }
+  ]
+}