compiler: Return a single column from coalesce (#639)

Fixes #458

Author: kyleconroy

internal/compiler/output_columns.go

@@ -97,18 +97,26 @@ func outputColumns(qc *QueryCatalog, node ast.Node) ([]*Column, error) {
 			}
 
 		case *pg.CoalesceExpr:
+			var found bool
 			for _, arg := range n.Args.Items {
+				if found {
+					continue
+				}
 				if ref, ok := arg.(*pg.ColumnRef); ok {
 					columns, err := outputColumnRefs(res, tables, ref)
 					if err != nil {
 						return nil, err
 					}
 					for _, c := range columns {
+						found = true
 						c.NotNull = true
 						cols = append(cols, c)
 					}
 				}
 			}
+			if !found {
+				cols = append(cols, &Column{Name: "coalesce", DataType: "any", NotNull: false})
+			}
 
 		case *pg.ColumnRef:
 			if hasStarRef(n) {

internal/endtoend/testdata/coalesce/go/models.go

@@ -1,5 +1,9 @@
-CREATE TABLE foo (bar text);
+CREATE TABLE foo (bar text, bat text not null);
 
 -- name: Coalesce :many
 SELECT coalesce(bar, '') as login
 FROM foo;
+
+-- name: CoalesceColumns :many
+SELECT bar, bat, coalesce(bar, bat)
+FROM foo;