doug-martin#204: support exists expression

* add exists expression

* add select_dataset methods

* exists works for non prepared queries

* exists works for non prepared queries

* fixed test with prepared statement

* more expression examples

* expression examples fix

expressions.go

@@ -328,6 +328,10 @@ func All(val interface{}) exp.SQLFunctionExpression {
 	return Func("ALL ", val)
 }
 
+func Exists(val interface{}) exp.SQLFunctionExpression {
+	return Func("EXISTS ", val)
+}
+
 func Case() exp.CaseExpression {
 	return exp.NewCaseExpression()
 }

expressions_example_test.go

@@ -1807,6 +1807,32 @@ func ExampleAny() {
 	// SELECT * FROM "test" WHERE ("id" = ANY ((SELECT "test_id" FROM "other"))) []
 }
 
+func ExampleExists() {
+	ds := goqu.Select(goqu.Exists(goqu.From("test")))
+	sql, args, _ := ds.ToSQL()
+	fmt.Println(sql, args)
+
+	sql, args, _ = ds.Prepared(true).ToSQL()
+	fmt.Println(sql, args)
+	// Output:
+	// SELECT EXISTS ((SELECT * FROM "test")) []
+	// SELECT EXISTS ((SELECT * FROM "test")) []
+}
+
+func ExampleExists_inWhereClauses() {
+	ds := goqu.From("test").Where(
+		goqu.Exists(goqu.From("other").Where(goqu.Ex{"test_id": goqu.I("id")})),
+	)
+	sql, args, _ := ds.ToSQL()
+	fmt.Println(sql, args)
+
+	sql, args, _ = ds.Prepared(true).ToSQL()
+	fmt.Println(sql, args)
+	// Output:
+	// SELECT * FROM "test" WHERE EXISTS ((SELECT * FROM "other" WHERE ("test_id" = "id"))) []
+	// SELECT * FROM "test" WHERE EXISTS ((SELECT * FROM "other" WHERE ("test_id" = "id"))) []
+}
+
 func ExampleAll() {
 	ds := goqu.From("test").Where(goqu.Ex{
 		"id": goqu.All(goqu.From("other").Select("test_id")),

expressions_test.go

@@ -185,6 +185,13 @@ func (ges *goquExpressionsSuite) TestAll() {
 	ges.Equal(exp.NewSQLFunctionExpression("ALL ", ds), goqu.All(ds))
 }
 
+func (ges *goquExpressionsSuite) TestExists() {
+	ds := goqu.From("test").Where(
+		goqu.Exists(goqu.From("other").Where(goqu.Ex{"test_id": goqu.I("id")})),
+	)
+	ges.Equal(exp.NewSQLFunctionExpression("EXISTS ", ds), goqu.Exists(ds))
+}
+
 func TestGoquExpressions(t *testing.T) {
 	suite.Run(t, new(goquExpressionsSuite))
 }

select_dataset.go

@@ -671,6 +671,18 @@ func (sd *SelectDataset) CountContext(ctx context.Context) (int64, error) {
 	return count, err
 }
 
+// Generates the SELECT EXISTS(...) sql for this dataset and uses Exec#ScanVal to scan the result into an bool.
+func (sd *SelectDataset) Exists() (bool, error) {
+	return sd.ExistsContext(context.Background())
+}
+
+// Generates the SELECT EXISTS(...) sql for this dataset and uses Exec#ScanValContext to scan the result into an bool.
+func (sd *SelectDataset) ExistsContext(ctx context.Context) (bool, error) {
+	var exists bool
+	_, err := newDataset(sd.dialect.Dialect(), sd.queryFactory).Select(Exists(sd)).Prepared(sd.IsPrepared()).ScanValContext(ctx, &exists)
+	return exists, err
+}
+
 // Generates the SELECT sql only selecting the passed in column and uses Exec#ScanVals to scan the result into a slice
 // of primitive values.
 //

select_dataset_example_test.go

@@ -1557,6 +1557,18 @@ func ExampleSelectDataset_Count() {
 	// Count is 4
 }
 
+func ExampleSelectDataset_Exists() {
+	exists, err := getDB().From("goqu_user").Exists()
+	if err != nil {
+		fmt.Println(err.Error())
+		return
+	}
+	fmt.Printf("Exists is %t", exists)
+
+	// Output:
+	// Exists is true
+}
+
 func ExampleSelectDataset_Pluck() {
 	var lastNames []string
 	if err := getDB().From("goqu_user").Pluck(&lastNames, "last_name"); err != nil {

select_dataset_test.go

@@ -1543,6 +1543,37 @@ func (sds *selectDatasetSuite) TestCount_WithPreparedStatement() {
 	sds.Equal(int64(10), count)
 }
 
+func (sds *selectDatasetSuite) TestExists() {
+	mDB, sqlMock, err := sqlmock.New()
+	sds.NoError(err)
+	sqlMock.ExpectQuery(`SELECT EXISTS \(\(SELECT \* FROM "items"\)\) LIMIT 1`).
+		WithArgs().
+		WillReturnRows(sqlmock.NewRows([]string{"exists"}).FromCSVString("true"))
+
+	db := goqu.New("mock", mDB)
+	exists, err := db.From("items").Exists()
+	sds.NoError(err)
+	sds.True(exists)
+}
+
+func (sds *selectDatasetSuite) TestExists_WithPreparedStatement() {
+	mDB, sqlMock, err := sqlmock.New()
+	sds.NoError(err)
+	sqlMock.ExpectQuery(
+		`SELECT EXISTS \(\(SELECT \* FROM "items" WHERE \("address" = \?\)\)\) LIMIT \?`,
+	).
+		WithArgs("111 Test Addr", 1).
+		WillReturnRows(sqlmock.NewRows([]string{"exists"}).FromCSVString("true"))
+
+	ds := goqu.New("mock", mDB)
+	exists, err := ds.From("items").
+		Prepared(true).
+		Where(goqu.Ex{"address": "111 Test Addr"}).
+		Exists()
+	sds.NoError(err)
+	sds.True(exists)
+}
+
 func (sds *selectDatasetSuite) TestPluck() {
 	mDB, sqlMock, err := sqlmock.New()
 	sds.NoError(err)