Enhancement for #119

* [Added] Support for `DISTINCT ON` clauses #119

HISTORY.md

@@ -1,3 +1,7 @@
+## 8.3.0
+
+* [Added] Support for `DISTINCT ON` clauses [#119](https://github.com/doug-martin/goqu/issues/119)
+
 ## v8.2.2
 
 * [FIX] Scanner errors on pointers to primitive values [#122](https://github.com/doug-martin/goqu/issues/122)

dialect/mysql/mysql.go

@@ -18,6 +18,7 @@ func DialectOptions() *goqu.SQLDialectOptions {
 	opts.SupportsConflictTarget = false
 	opts.SupportsWithCTE = false
 	opts.SupportsWithCTERecursive = false
+	opts.SupportsDistinctOn = false
 
 	opts.UseFromClauseForMultipleUpdateTables = false
 

dialect/sqlite3/sqlite3.go

@@ -18,6 +18,7 @@ func DialectOptions() *goqu.SQLDialectOptions {
 	opts.SupportsConflictTarget = false
 	opts.SupportsMultipleUpdateTables = false
 	opts.WrapCompoundsInParens = false
+	opts.SupportsDistinctOn = false
 
 	opts.PlaceHolderRune = '?'
 	opts.IncludePlaceholderNum = false

docs/selecting.md

@@ -3,7 +3,7 @@
 * [Creating a SelectDataset](#create)
 * Building SQL
   * [`Select`](#select)
-  * [`SelectDistinct`](#select_distinct)
+  * [`Distinct`](#distinct)
   * [`From`](#from)
   * [`Join`](#joins)
   * [`Where`](#where)
@@ -162,11 +162,11 @@ Output:
 SELECT "address", "email_address", "name" FROM "test"
 ```
 
-<a name="select_distinct"></a>
-**[`SelectDistinct`](https://godoc.org/github.com/doug-martin/goqu/#SelectDataset.SelectDistinct)**
+<a name="distinct"></a>
+**[`Distinct`](https://godoc.org/github.com/doug-martin/goqu/#SelectDataset.Distinct)**
 
 ```go
-sql, _, _ := goqu.From("test").SelectDistinct("a", "b").ToSQL()
+sql, _, _ := goqu.From("test").Select("a", "b").Distinct().ToSQL()
 fmt.Println(sql)
 ```
 
@@ -175,6 +175,42 @@ Output:
 SELECT DISTINCT "a", "b" FROM "test"
 ```
 
+If you dialect supports `DISTINCT ON` you provide arguments to the `Distinct` method.
+
+**NOTE** currently only the `postgres` and the default dialects support `DISTINCT ON` clauses
+
+```go
+sql, _, _ := goqu.From("test").Distinct("a").ToSQL()
+fmt.Println(sql)
+```
+Output:
+
+```
+SELECT DISTINCT ON ("a") * FROM "test"
+```
+
+You can also provide other expression arguments
+
+With `goqu.L`
+
+```go
+sql, _, _ := goqu.From("test").Distinct(goqu.L("COALESCE(?, ?)", goqu.C("a"), "empty")).ToSQL()
+fmt.Println(sql)
+```
+Output:
+```
+SELECT DISTINCT ON (COALESCE("a", 'empty')) * FROM "test"
+```
+With `goqu.Coalesce`
+```go
+sql, _, _ := goqu.From("test").Distinct(goqu.COALESCE(goqu.C("a"), "empty")).ToSQL()
+fmt.Println(sql)
+```
+Output:
+```
+SELECT DISTINCT ON (COALESCE("a", 'empty')) * FROM "test"
+```
+
 <a name="from"></a>
 **[`From`](https://godoc.org/github.com/doug-martin/goqu/#SelectDataset.From)**
 

exp/select_clauses.go

@@ -10,9 +10,8 @@ type (
 		SelectAppend(cl ColumnListExpression) SelectClauses
 		SetSelect(cl ColumnListExpression) SelectClauses
 
-		SelectDistinct() ColumnListExpression
-		HasSelectDistinct() bool
-		SetSelectDistinct(cl ColumnListExpression) SelectClauses
+		Distinct() ColumnListExpression
+		SetDistinct(cle ColumnListExpression) SelectClauses
 
 		From() ColumnListExpression
 		SetFrom(cl ColumnListExpression) SelectClauses
@@ -61,20 +60,20 @@ type (
 		CommonTablesAppend(cte CommonTableExpression) SelectClauses
 	}
 	selectClauses struct {
-		commonTables   []CommonTableExpression
-		selectColumns  ColumnListExpression
-		selectDistinct ColumnListExpression
-		from           ColumnListExpression
-		joins          JoinExpressions
-		where          ExpressionList
-		alias          IdentifierExpression
-		groupBy        ColumnListExpression
-		having         ExpressionList
-		order          ColumnListExpression
-		limit          interface{}
-		offset         uint
-		compounds      []CompoundExpression
-		lock           Lock
+		commonTables  []CommonTableExpression
+		selectColumns ColumnListExpression
+		distinct      ColumnListExpression
+		from          ColumnListExpression
+		joins         JoinExpressions
+		where         ExpressionList
+		alias         IdentifierExpression
+		groupBy       ColumnListExpression
+		having        ExpressionList
+		order         ColumnListExpression
+		limit         interface{}
+		offset        uint
+		compounds     []CompoundExpression
+		lock          Lock
 	}
 )
 
@@ -103,20 +102,20 @@ func (c *selectClauses) IsDefaultSelect() bool {
 
 func (c *selectClauses) clone() *selectClauses {
 	return &selectClauses{
-		commonTables:   c.commonTables,
-		selectColumns:  c.selectColumns,
-		selectDistinct: c.selectDistinct,
-		from:           c.from,
-		joins:          c.joins,
-		where:          c.where,
-		alias:          c.alias,
-		groupBy:        c.groupBy,
-		having:         c.having,
-		order:          c.order,
-		limit:          c.limit,
-		offset:         c.offset,
-		compounds:      c.compounds,
-		lock:           c.lock,
+		commonTables:  c.commonTables,
+		selectColumns: c.selectColumns,
+		distinct:      c.distinct,
+		from:          c.from,
+		joins:         c.joins,
+		where:         c.where,
+		alias:         c.alias,
+		groupBy:       c.groupBy,
+		having:        c.having,
+		order:         c.order,
+		limit:         c.limit,
+		offset:        c.offset,
+		compounds:     c.compounds,
+		lock:          c.lock,
 	}
 }
 
@@ -134,31 +133,22 @@ func (c *selectClauses) Select() ColumnListExpression {
 }
 func (c *selectClauses) SelectAppend(cl ColumnListExpression) SelectClauses {
 	ret := c.clone()
-	if ret.selectDistinct != nil {
-		ret.selectDistinct = ret.selectDistinct.Append(cl.Columns()...)
-	} else {
-		ret.selectColumns = ret.selectColumns.Append(cl.Columns()...)
-	}
+	ret.selectColumns = ret.selectColumns.Append(cl.Columns()...)
 	return ret
 }
 
 func (c *selectClauses) SetSelect(cl ColumnListExpression) SelectClauses {
 	ret := c.clone()
-	ret.selectDistinct = nil
 	ret.selectColumns = cl
 	return ret
 }
 
-func (c *selectClauses) SelectDistinct() ColumnListExpression {
-	return c.selectDistinct
-}
-func (c *selectClauses) HasSelectDistinct() bool {
-	return c.selectDistinct != nil
+func (c *selectClauses) Distinct() ColumnListExpression {
+	return c.distinct
 }
-func (c *selectClauses) SetSelectDistinct(cl ColumnListExpression) SelectClauses {
+func (c *selectClauses) SetDistinct(cle ColumnListExpression) SelectClauses {
 	ret := c.clone()
-	ret.selectColumns = nil
-	ret.selectDistinct = cl
+	ret.distinct = cle
 	return ret
 }
 

exp/select_clauses_test.go

@@ -65,10 +65,7 @@ func (sct *selectClausesTest) TestSelectAppend() {
 	c2 := c.SelectAppend(NewColumnListExpression("a"))
 
 	assert.Equal(t, NewColumnListExpression(Star()), c.Select())
-	assert.Nil(t, c.SelectDistinct())
-
 	assert.Equal(t, NewColumnListExpression(Star(), "a"), c2.Select())
-	assert.Nil(t, c2.SelectDistinct())
 }
 
 func (sct *selectClausesTest) TestSetSelect() {
@@ -77,35 +74,32 @@ func (sct *selectClausesTest) TestSetSelect() {
 	c2 := c.SetSelect(NewColumnListExpression("a"))
 
 	assert.Equal(t, NewColumnListExpression(Star()), c.Select())
-	assert.Nil(t, c.SelectDistinct())
-
 	assert.Equal(t, NewColumnListExpression("a"), c2.Select())
-	assert.Nil(t, c2.SelectDistinct())
 
 }
 
-func (sct *selectClausesTest) TestSelectDistinct() {
+func (sct *selectClausesTest) TestDistinct() {
 	t := sct.T()
 	c := NewSelectClauses()
-	c2 := c.SetSelectDistinct(NewColumnListExpression("a"))
+	c2 := c.SetDistinct(NewColumnListExpression("a"))
 
-	assert.Nil(t, c.SelectDistinct())
+	assert.Nil(t, c.Distinct())
 	assert.Equal(t, NewColumnListExpression(Star()), c.Select())
 
-	assert.Equal(t, NewColumnListExpression("a"), c2.SelectDistinct())
-	assert.Nil(t, c2.Select())
+	assert.Equal(t, NewColumnListExpression("a"), c2.Distinct())
+	assert.Equal(t, NewColumnListExpression(Star()), c.Select())
 }
 
 func (sct *selectClausesTest) TestSetSelectDistinct() {
 	t := sct.T()
 	c := NewSelectClauses()
-	c2 := c.SetSelectDistinct(NewColumnListExpression("a"))
+	c2 := c.SetDistinct(NewColumnListExpression("a"))
 
-	assert.Nil(t, c.SelectDistinct())
+	assert.Nil(t, c.Distinct())
 	assert.Equal(t, NewColumnListExpression(Star()), c.Select())
 
-	assert.Equal(t, NewColumnListExpression("a"), c2.SelectDistinct())
-	assert.Nil(t, c2.Select())
+	assert.Equal(t, NewColumnListExpression("a"), c2.Distinct())
+	assert.Equal(t, NewColumnListExpression(Star()), c.Select())
 }
 
 func (sct *selectClausesTest) TestFrom() {

select_dataset.go

@@ -223,14 +223,15 @@ func (sd *SelectDataset) Select(selects ...interface{}) *SelectDataset {
 //   SQLFunction: (See Func, MIN, MAX, COUNT....)
 //   Struct: If passing in an instance of a struct, we will parse the struct for the column names to select.
 //   See examples
+// Deprecated: Use Distinct() instead.
 func (sd *SelectDataset) SelectDistinct(selects ...interface{}) *SelectDataset {
-	return sd.copy(sd.clauses.SetSelectDistinct(exp.NewColumnListExpression(selects...)))
+	return sd.copy(sd.clauses.SetSelect(exp.NewColumnListExpression(selects...)).SetDistinct(exp.NewColumnListExpression()))
 }
 
-// Resets to SELECT *. If the SelectDistinct was used the returned Dataset will have the the dataset set to SELECT *.
+// Resets to SELECT *. If the SelectDistinct or Distinct was used the returned Dataset will have the the dataset set to SELECT *.
 // See examples.
 func (sd *SelectDataset) ClearSelect() *SelectDataset {
-	return sd.copy(sd.clauses.SetSelect(exp.NewColumnListExpression(exp.Star())))
+	return sd.copy(sd.clauses.SetSelect(exp.NewColumnListExpression(exp.Star())).SetDistinct(nil))
 }
 
 // Adds columns to the SELECT clause. See examples
@@ -244,6 +245,10 @@ func (sd *SelectDataset) SelectAppend(selects ...interface{}) *SelectDataset {
 	return sd.copy(sd.clauses.SelectAppend(exp.NewColumnListExpression(selects...)))
 }
 
+func (sd *SelectDataset) Distinct(on ...interface{}) *SelectDataset {
+	return sd.copy(sd.clauses.SetDistinct(exp.NewColumnListExpression(on...)))
+}
+
 // Adds a FROM clause. This return a new dataset with the original sources replaced. See examples.
 // You can pass in the following.
 //   string: Will automatically be turned into an identifier

select_dataset_example_test.go

@@ -939,18 +939,39 @@ func ExampleSelectDataset_Select_withStruct() {
 	// SELECT "address", "email_address", "name" FROM "test"
 }
 
-func ExampleSelectDataset_SelectDistinct() {
-	sql, _, _ := goqu.From("test").SelectDistinct("a", "b").ToSQL()
+func ExampleSelectDataset_Distinct() {
+	sql, _, _ := goqu.From("test").Select("a", "b").Distinct().ToSQL()
 	fmt.Println(sql)
 	// Output:
 	// SELECT DISTINCT "a", "b" FROM "test"
 }
 
+func ExampleSelectDataset_Distinct_on() {
+	sql, _, _ := goqu.From("test").Distinct("a").ToSQL()
+	fmt.Println(sql)
+	// Output:
+	// SELECT DISTINCT ON ("a") * FROM "test"
+}
+
+func ExampleSelectDataset_Distinct_onWithLiteral() {
+	sql, _, _ := goqu.From("test").Distinct(goqu.L("COALESCE(?, ?)", goqu.C("a"), "empty")).ToSQL()
+	fmt.Println(sql)
+	// Output:
+	// SELECT DISTINCT ON (COALESCE("a", 'empty')) * FROM "test"
+}
+
+func ExampleSelectDataset_Distinct_onCoalesce() {
+	sql, _, _ := goqu.From("test").Distinct(goqu.COALESCE(goqu.C("a"), "empty")).ToSQL()
+	fmt.Println(sql)
+	// Output:
+	// SELECT DISTINCT ON (COALESCE("a", 'empty')) * FROM "test"
+}
+
 func ExampleSelectDataset_SelectAppend() {
 	ds := goqu.From("test").Select("a", "b")
 	sql, _, _ := ds.SelectAppend("c").ToSQL()
 	fmt.Println(sql)
-	ds = goqu.From("test").SelectDistinct("a", "b")
+	ds = goqu.From("test").Select("a", "b").Distinct()
 	sql, _, _ = ds.SelectAppend("c").ToSQL()
 	fmt.Println(sql)
 	// Output:
@@ -962,7 +983,7 @@ func ExampleSelectDataset_ClearSelect() {
 	ds := goqu.From("test").Select("a", "b")
 	sql, _, _ := ds.ClearSelect().ToSQL()
 	fmt.Println(sql)
-	ds = goqu.From("test").SelectDistinct("a", "b")
+	ds = goqu.From("test").Select("a", "b").Distinct()
 	sql, _, _ = ds.ClearSelect().ToSQL()
 	fmt.Println(sql)
 	// Output: