Generate nullable value from subselect statements

internal/compiler/output_columns.go

@@ -3,6 +3,7 @@ package compiler
 import (
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/sqlc-dev/sqlc/internal/sql/ast"
 	"github.com/sqlc-dev/sqlc/internal/sql/astutils"
@@ -315,12 +316,14 @@ func (c *Compiler) outputColumns(qc *QueryCatalog, node ast.Node) ([]*Column, er
 					DataType:   dataType(fun.ReturnType),
 					NotNull:    !fun.ReturnTypeNullable,
 					IsFuncCall: true,
+					FuncName:   rel.Name,
 				})
 			} else {
 				cols = append(cols, &Column{
 					Name:       name,
 					DataType:   "any",
 					IsFuncCall: true,
+					FuncName:   rel.Name,
 				})
 			}
 
@@ -341,6 +344,10 @@ func (c *Compiler) outputColumns(qc *QueryCatalog, node ast.Node) ([]*Column, er
 				if res.Name != nil {
 					first.Name = *res.Name
 				}
+				if !(first.IsFuncCall && strings.EqualFold(first.FuncName, "count")) {
+					first.NotNull = false
+				}
+
 				cols = append(cols, first)
 			default:
 				cols = append(cols, &Column{Name: name, DataType: "any", NotNull: false})
@@ -377,6 +384,12 @@ func (c *Compiler) outputColumns(qc *QueryCatalog, node ast.Node) ([]*Column, er
 			if res.Name != nil {
 				first.Name = *res.Name
 			}
+			if !(first.IsFuncCall &&
+				(strings.EqualFold(first.FuncName, "count") ||
+					strings.EqualFold(first.FuncName, "total"))) {
+				first.NotNull = false
+			}
+
 			cols = append(cols, first)
 
 		default:

internal/compiler/query.go

@@ -29,6 +29,7 @@ type Column struct {
 	Length       *int
 	IsNamedParam bool
 	IsFuncCall   bool
+	FuncName     string
 
 	// XXX: Figure out what PostgreSQL calls `foo.id`
 	Scope      string

internal/endtoend/testdata/nullable_subselect/mysql/query.sql

@@ -0,0 +1,10 @@
+-- name: FirstRowFromFooTable :many
+SELECT a, (SELECT a FROM foo limit 1) as "first" FROM foo;
+
+-- name: FirstRowFromEmptyTable :many
+SELECT a, (SELECT a FROM empty limit 1) as "first" FROM foo;
+
+-- In MySQL, only count() returns 0 for empty table.
+-- https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html
+-- name: CountRowsEmptyTable :many
+SELECT a, (SELECT count(a) FROM empty) as "count" FROM foo;

internal/endtoend/testdata/nullable_subselect/mysql/schema.sql

@@ -0,0 +1,7 @@
+CREATE TABLE foo (a int not NULL, b int);
+
+INSERT INTO foo VALUES (1, 2);
+INSERT INTO foo VALUES (3, NULL);
+INSERT INTO foo VALUES (4, 5);
+
+CREATE TABLE empty (a int not NULL, b int);

internal/endtoend/testdata/nullable_subselect/mysql/sqlc.json

@@ -0,0 +1,12 @@
+{
+  "version": "1",
+  "packages": [
+    {
+      "path": "go",
+      "engine": "mysql",
+      "name": "querytest",
+      "schema": "schema.sql",
+      "queries": "query.sql"
+    }
+  ]
+}