Add Distinct support

callbacks/query.go

@@ -37,7 +37,7 @@ func Query(db *gorm.DB) {
 }
 
 func BuildQuerySQL(db *gorm.DB) {
-	clauseSelect := clause.Select{}
+	clauseSelect := clause.Select{Distinct: db.Statement.Distinct}
 
 	if db.Statement.ReflectValue.Kind() == reflect.Struct {
 		var conds []clause.Expression

chainable_api.go

@@ -45,6 +45,16 @@ func (db *DB) Table(name string) (tx *DB) {
 	return
 }
 
+// Distinct specify distinct fields that you want querying
+func (db *DB) Distinct(args ...interface{}) (tx *DB) {
+	tx = db
+	if len(args) > 0 {
+		tx = tx.Select(args[0], args[1:]...)
+	}
+	tx.Statement.Distinct = true
+	return tx
+}
+
 // Select specify fields that you want when querying, creating, updating
 func (db *DB) Select(query interface{}, args ...interface{}) (tx *DB) {
 	tx = db.getInstance()

clause/select.go

@@ -2,6 +2,7 @@ package clause
 
 // Select select attrs when querying, updating, creating
 type Select struct {
+	Distinct   bool
 	Columns    []Column
 	Expression Expression
 }
@@ -12,6 +13,10 @@ func (s Select) Name() string {
 
 func (s Select) Build(builder Builder) {
 	if len(s.Columns) > 0 {
+		if s.Distinct {
+			builder.WriteString(" DISTINCT ")
+		}
+
 		for idx, column := range s.Columns {
 			if idx > 0 {
 				builder.WriteByte(',')

errors.go

@@ -23,4 +23,6 @@ var (
 	ErrPtrStructSupported = errors.New("only ptr of struct supported")
 	// ErrorPrimaryKeyRequired primary keys required
 	ErrorPrimaryKeyRequired = errors.New("primary key required")
+	// ErrorModelValueRequired model value required
+	ErrorModelValueRequired = errors.New("model value required")
 )

finisher_api.go

@@ -233,13 +233,24 @@ func (db *DB) Delete(value interface{}, conds ...interface{}) (tx *DB) {
 
 func (db *DB) Count(count *int64) (tx *DB) {
 	tx = db.getInstance()
-	if s, ok := tx.Statement.Clauses["SELECT"].Expression.(clause.Select); !ok || len(s.Columns) == 0 {
-		tx.Statement.AddClause(clause.Select{Expression: clause.Expr{SQL: "count(1)"}})
-	}
-
 	if tx.Statement.Model == nil {
 		tx.Statement.Model = tx.Statement.Dest
 	}
+
+	if len(tx.Statement.Selects) == 0 {
+		tx.Statement.AddClause(clause.Select{Expression: clause.Expr{SQL: "count(1)"}})
+	} else if len(tx.Statement.Selects) == 1 && !strings.Contains(strings.ToLower(tx.Statement.Selects[0]), "count(") {
+		column := tx.Statement.Selects[0]
+		if tx.Statement.Parse(tx.Statement.Model) == nil {
+			if f := tx.Statement.Schema.LookUpField(column); f != nil {
+				column = f.DBName
+			}
+		}
+		tx.Statement.AddClause(clause.Select{
+			Expression: clause.Expr{SQL: "COUNT(DISTINCT(?))", Vars: []interface{}{clause.Column{Name: column}}},
+		})
+	}
+
 	tx.Statement.Dest = count
 	tx.callbacks.Query().Execute(tx)
 	if db.RowsAffected != 1 {
@@ -273,9 +284,22 @@ func (db *DB) Scan(dest interface{}) (tx *DB) {
 //     db.Find(&users).Pluck("age", &ages)
 func (db *DB) Pluck(column string, dest interface{}) (tx *DB) {
 	tx = db.getInstance()
-	tx.Statement.AddClauseIfNotExists(clause.Select{Columns: []clause.Column{{Name: column}}})
+	if tx.Statement.Model != nil {
+		if tx.Statement.Parse(tx.Statement.Model) == nil {
+			if f := tx.Statement.Schema.LookUpField(column); f != nil {
+				column = f.DBName
+			}
+		}
+
+		tx.Statement.AddClauseIfNotExists(clause.Select{
+			Distinct: tx.Statement.Distinct,
+			Columns:  []clause.Column{{Name: column}},
+		})
 		tx.Statement.Dest = dest
 		tx.callbacks.Query().Execute(tx)
+	} else {
+		tx.AddError(ErrorModelValueRequired)
+	}
 	return
 }
 

statement.go

@@ -23,6 +23,7 @@ type Statement struct {
 	Dest                 interface{}
 	ReflectValue         reflect.Value
 	Clauses              map[string]clause.Clause
+	Distinct             bool
 	Selects              []string // selected columns
 	Omits                []string // omit columns
 	Joins                map[string][]interface{}
@@ -331,6 +332,7 @@ func (stmt *Statement) clone() *Statement {
 		Dest:                 stmt.Dest,
 		ReflectValue:         stmt.ReflectValue,
 		Clauses:              map[string]clause.Clause{},
+		Distinct:             stmt.Distinct,
 		Selects:              stmt.Selects,
 		Omits:                stmt.Omits,
 		Joins:                map[string][]interface{}{},

tests/distinct_test.go

@@ -0,0 +1,60 @@
+package tests_test
+
+import (
+	"testing"
+
+	. "gorm.io/gorm/utils/tests"
+)
+
+func TestDistinct(t *testing.T) {
+	var users = []User{
+		*GetUser("distinct", Config{}),
+		*GetUser("distinct", Config{}),
+		*GetUser("distinct", Config{}),
+		*GetUser("distinct-2", Config{}),
+		*GetUser("distinct-3", Config{}),
+	}
+	users[0].Age = 20
+
+	if err := DB.Create(&users).Error; err != nil {
+		t.Fatalf("errors happened when create users: %v", err)
+	}
+
+	var names []string
+	DB.Model(&User{}).Where("name like ?", "distinct%").Order("name").Pluck("Name", &names)
+	AssertEqual(t, names, []string{"distinct", "distinct", "distinct", "distinct-2", "distinct-3"})
+
+	var names1 []string
+	DB.Model(&User{}).Where("name like ?", "distinct%").Distinct().Order("name").Pluck("Name", &names1)
+
+	AssertEqual(t, names1, []string{"distinct", "distinct-2", "distinct-3"})
+
+	var results []User
+	if err := DB.Distinct("name", "age").Where("name like ?", "distinct%").Order("name, age desc").Find(&results).Error; err != nil {
+		t.Errorf("failed to query users, got error: %v", err)
+	}
+
+	expects := []User{
+		{Name: "distinct", Age: 20},
+		{Name: "distinct", Age: 18},
+		{Name: "distinct-2", Age: 18},
+		{Name: "distinct-3", Age: 18},
+	}
+
+	if len(results) != 4 {
+		t.Fatalf("invalid results length found, expects: %v, got %v", len(expects), len(results))
+	}
+
+	for idx, expect := range expects {
+		AssertObjEqual(t, results[idx], expect, "Name", "Age")
+	}
+
+	var count int64
+	if err := DB.Model(&User{}).Where("name like ?", "distinct%").Count(&count).Error; err != nil || count != 5 {
+		t.Errorf("failed to query users count, got error: %v, count: %v", err, count)
+	}
+
+	if err := DB.Model(&User{}).Distinct("name").Where("name like ?", "distinct%").Count(&count).Error; err != nil || count != 3 {
+		t.Errorf("failed to query users count, got error: %v, count %v", err, count)
+	}
+}