Merge pull request #116 from huandu/feature-generate-sql-only-when-value-is-set

Fix #115: Generate SQL only when necessary values are set

args.go

@@ -97,7 +97,7 @@ func (args *Args) Compile(format string, initialValue ...interface{}) (query str
 //
 // See doc for `Compile` to learn details.
 func (args *Args) CompileWithFlavor(format string, flavor Flavor, initialValue ...interface{}) (query string, values []interface{}) {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	idx := strings.IndexRune(format, '$')
 	offset := 0
 	values = initialValue
@@ -160,7 +160,7 @@ func (args *Args) CompileWithFlavor(format string, flavor Flavor, initialValue .
 	return
 }
 
-func (args *Args) compileNamed(buf *strings.Builder, flavor Flavor, format string, values []interface{}) (string, []interface{}) {
+func (args *Args) compileNamed(buf *stringBuilder, flavor Flavor, format string, values []interface{}) (string, []interface{}) {
 	i := 1
 
 	for ; i < len(format) && format[i] != '}'; i++ {
@@ -182,7 +182,7 @@ func (args *Args) compileNamed(buf *strings.Builder, flavor Flavor, format strin
 	return format, values
 }
 
-func (args *Args) compileDigits(buf *strings.Builder, flavor Flavor, format string, values []interface{}, offset int) (string, []interface{}, int) {
+func (args *Args) compileDigits(buf *stringBuilder, flavor Flavor, format string, values []interface{}, offset int) (string, []interface{}, int) {
 	i := 1
 
 	for ; i < len(format) && '0' <= format[i] && format[i] <= '9'; i++ {
@@ -199,7 +199,7 @@ func (args *Args) compileDigits(buf *strings.Builder, flavor Flavor, format stri
 	return format, values, offset
 }
 
-func (args *Args) compileSuccessive(buf *strings.Builder, flavor Flavor, format string, values []interface{}, offset int) (string, []interface{}, int) {
+func (args *Args) compileSuccessive(buf *stringBuilder, flavor Flavor, format string, values []interface{}, offset int) (string, []interface{}, int) {
 	if offset >= len(args.args) {
 		return format, values, offset
 	}
@@ -210,7 +210,7 @@ func (args *Args) compileSuccessive(buf *strings.Builder, flavor Flavor, format
 	return format, values, offset + 1
 }
 
-func (args *Args) compileArg(buf *strings.Builder, flavor Flavor, values []interface{}, arg interface{}) []interface{} {
+func (args *Args) compileArg(buf *stringBuilder, flavor Flavor, values []interface{}, arg interface{}) []interface{} {
 	switch a := arg.(type) {
 	case Builder:
 		var s string

cond.go

@@ -14,7 +14,7 @@ type Cond struct {
 
 // Equal represents "field = value".
 func (c *Cond) Equal(field string, value interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" = ")
 	buf.WriteString(c.Args.Add(value))
@@ -28,7 +28,7 @@ func (c *Cond) E(field string, value interface{}) string {
 
 // NotEqual represents "field <> value".
 func (c *Cond) NotEqual(field string, value interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" <> ")
 	buf.WriteString(c.Args.Add(value))
@@ -42,7 +42,7 @@ func (c *Cond) NE(field string, value interface{}) string {
 
 // GreaterThan represents "field > value".
 func (c *Cond) GreaterThan(field string, value interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" > ")
 	buf.WriteString(c.Args.Add(value))
@@ -56,7 +56,7 @@ func (c *Cond) G(field string, value interface{}) string {
 
 // GreaterEqualThan represents "field >= value".
 func (c *Cond) GreaterEqualThan(field string, value interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" >= ")
 	buf.WriteString(c.Args.Add(value))
@@ -70,7 +70,7 @@ func (c *Cond) GE(field string, value interface{}) string {
 
 // LessThan represents "field < value".
 func (c *Cond) LessThan(field string, value interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" < ")
 	buf.WriteString(c.Args.Add(value))
@@ -84,7 +84,7 @@ func (c *Cond) L(field string, value interface{}) string {
 
 // LessEqualThan represents "field <= value".
 func (c *Cond) LessEqualThan(field string, value interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" <= ")
 	buf.WriteString(c.Args.Add(value))
@@ -104,7 +104,7 @@ func (c *Cond) In(field string, value ...interface{}) string {
 		vs = append(vs, c.Args.Add(v))
 	}
 
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" IN (")
 	buf.WriteString(strings.Join(vs, ", "))
@@ -120,7 +120,7 @@ func (c *Cond) NotIn(field string, value ...interface{}) string {
 		vs = append(vs, c.Args.Add(v))
 	}
 
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" NOT IN (")
 	buf.WriteString(strings.Join(vs, ", "))
@@ -130,7 +130,7 @@ func (c *Cond) NotIn(field string, value ...interface{}) string {
 
 // Like represents "field LIKE value".
 func (c *Cond) Like(field string, value interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" LIKE ")
 	buf.WriteString(c.Args.Add(value))
@@ -139,7 +139,7 @@ func (c *Cond) Like(field string, value interface{}) string {
 
 // NotLike represents "field NOT LIKE value".
 func (c *Cond) NotLike(field string, value interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" NOT LIKE ")
 	buf.WriteString(c.Args.Add(value))
@@ -148,7 +148,7 @@ func (c *Cond) NotLike(field string, value interface{}) string {
 
 // IsNull represents "field IS NULL".
 func (c *Cond) IsNull(field string) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" IS NULL")
 	return buf.String()
@@ -156,7 +156,7 @@ func (c *Cond) IsNull(field string) string {
 
 // IsNotNull represents "field IS NOT NULL".
 func (c *Cond) IsNotNull(field string) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" IS NOT NULL")
 	return buf.String()
@@ -164,7 +164,7 @@ func (c *Cond) IsNotNull(field string) string {
 
 // Between represents "field BETWEEN lower AND upper".
 func (c *Cond) Between(field string, lower, upper interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" BETWEEN ")
 	buf.WriteString(c.Args.Add(lower))
@@ -175,7 +175,7 @@ func (c *Cond) Between(field string, lower, upper interface{}) string {
 
 // NotBetween represents "field NOT BETWEEN lower AND upper".
 func (c *Cond) NotBetween(field string, lower, upper interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" NOT BETWEEN ")
 	buf.WriteString(c.Args.Add(lower))
@@ -186,7 +186,7 @@ func (c *Cond) NotBetween(field string, lower, upper interface{}) string {
 
 // Or represents OR logic like "expr1 OR expr2 OR expr3".
 func (c *Cond) Or(orExpr ...string) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString("(")
 	buf.WriteString(strings.Join(orExpr, " OR "))
 	buf.WriteString(")")
@@ -195,7 +195,7 @@ func (c *Cond) Or(orExpr ...string) string {
 
 // And represents AND logic like "expr1 AND expr2 AND expr3".
 func (c *Cond) And(andExpr ...string) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString("(")
 	buf.WriteString(strings.Join(andExpr, " AND "))
 	buf.WriteString(")")
@@ -204,7 +204,7 @@ func (c *Cond) And(andExpr ...string) string {
 
 // Exists represents "EXISTS (subquery)".
 func (c *Cond) Exists(subquery interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString("EXISTS (")
 	buf.WriteString(c.Args.Add(subquery))
 	buf.WriteString(")")
@@ -213,7 +213,7 @@ func (c *Cond) Exists(subquery interface{}) string {
 
 // NotExists represents "NOT EXISTS (subquery)".
 func (c *Cond) NotExists(subquery interface{}) string {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString("NOT EXISTS (")
 	buf.WriteString(c.Args.Add(subquery))
 	buf.WriteString(")")
@@ -228,7 +228,7 @@ func (c *Cond) Any(field, op string, value ...interface{}) string {
 		vs = append(vs, c.Args.Add(v))
 	}
 
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" ")
 	buf.WriteString(op)
@@ -246,7 +246,7 @@ func (c *Cond) All(field, op string, value ...interface{}) string {
 		vs = append(vs, c.Args.Add(v))
 	}
 
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" ")
 	buf.WriteString(op)
@@ -264,7 +264,7 @@ func (c *Cond) Some(field, op string, value ...interface{}) string {
 		vs = append(vs, c.Args.Add(v))
 	}
 
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	buf.WriteString(Escape(field))
 	buf.WriteString(" ")
 	buf.WriteString(op)

createtable.go

@@ -100,20 +100,25 @@ func (ctb *CreateTableBuilder) Build() (sql string, args []interface{}) {
 // BuildWithFlavor returns compiled CREATE TABLE string and args with flavor and initial args.
 // They can be used in `DB#Query` of package `database/sql` directly.
 func (ctb *CreateTableBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{}) (sql string, args []interface{}) {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	ctb.injection.WriteTo(buf, createTableMarkerInit)
-	buf.WriteString(ctb.verb)
 
+	if len(ctb.verb) > 0 {
+		buf.WriteLeadingString(ctb.verb)
+	}
+
 	if ctb.ifNotExists {
-		buf.WriteString(" IF NOT EXISTS")
+		buf.WriteLeadingString("IF NOT EXISTS")
+	}
+
+	if len(ctb.table) > 0 {
+		buf.WriteLeadingString(ctb.table)
 	}
 
-	buf.WriteRune(' ')
-	buf.WriteString(ctb.table)
 	ctb.injection.WriteTo(buf, createTableMarkerAfterCreate)
 
 	if len(ctb.defs) > 0 {
-		buf.WriteString(" (")
+		buf.WriteLeadingString("(")
 
 		defs := make([]string, 0, len(ctb.defs))
 
@@ -128,15 +133,13 @@ func (ctb *CreateTableBuilder) BuildWithFlavor(flavor Flavor, initialArg ...inte
 	}
 
 	if len(ctb.options) > 0 {
-		buf.WriteRune(' ')
-
 		opts := make([]string, 0, len(ctb.options))
 
 		for _, opt := range ctb.options {
 			opts = append(opts, strings.Join(opt, " "))
 		}
 
-		buf.WriteString(strings.Join(opts, ", "))
+		buf.WriteLeadingString(strings.Join(opts, ", "))
 		ctb.injection.WriteTo(buf, createTableMarkerAfterOption)
 	}
 

delete.go

@@ -113,21 +113,25 @@ func (db *DeleteBuilder) Build() (sql string, args []interface{}) {
 // BuildWithFlavor returns compiled DELETE string and args with flavor and initial args.
 // They can be used in `DB#Query` of package `database/sql` directly.
 func (db *DeleteBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{}) (sql string, args []interface{}) {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	db.injection.WriteTo(buf, deleteMarkerInit)
-	buf.WriteString("DELETE FROM ")
-	buf.WriteString(db.table)
+
+	if len(db.table) > 0 {
+		buf.WriteLeadingString("DELETE FROM ")
+		buf.WriteString(db.table)
+	}
+
 	db.injection.WriteTo(buf, deleteMarkerAfterDeleteFrom)
 
 	if len(db.whereExprs) > 0 {
-		buf.WriteString(" WHERE ")
+		buf.WriteLeadingString("WHERE ")
 		buf.WriteString(strings.Join(db.whereExprs, " AND "))
 
 		db.injection.WriteTo(buf, deleteMarkerAfterWhere)
 	}
 
 	if len(db.orderByCols) > 0 {
-		buf.WriteString(" ORDER BY ")
+		buf.WriteLeadingString("ORDER BY ")
 		buf.WriteString(strings.Join(db.orderByCols, ", "))
 
 		if db.order != "" {
@@ -139,7 +143,7 @@ func (db *DeleteBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{
 	}
 
 	if db.limit >= 0 {
-		buf.WriteString(" LIMIT ")
+		buf.WriteLeadingString("LIMIT ")
 		buf.WriteString(strconv.Itoa(db.limit))
 
 		db.injection.WriteTo(buf, deleteMarkerAfterLimit)

injection.go

@@ -29,22 +29,13 @@ func (injection *injection) SQL(marker injectionMarker, sql string) {
 
 // WriteTo joins all SQL strings at the same marker value with blank (" ")
 // and writes the joined value to buf.
-func (injection *injection) WriteTo(buf *strings.Builder, marker injectionMarker) {
+func (injection *injection) WriteTo(buf *stringBuilder, marker injectionMarker) {
 	sqls := injection.markerSQLs[marker]
-	notEmpty := buf.Len() > 0
 
 	if len(sqls) == 0 {
 		return
 	}
 
-	if notEmpty {
-		buf.WriteRune(' ')
-	}
-
 	s := strings.Join(sqls, " ")
-	buf.WriteString(s)
-
-	if !notEmpty {
-		buf.WriteRune(' ')
-	}
+	buf.WriteLeadingString(s)
 }

insert.go

@@ -117,29 +117,36 @@ func (ib *InsertBuilder) Build() (sql string, args []interface{}) {
 // BuildWithFlavor returns compiled INSERT string and args with flavor and initial args.
 // They can be used in `DB#Query` of package `database/sql` directly.
 func (ib *InsertBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{}) (sql string, args []interface{}) {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	ib.injection.WriteTo(buf, insertMarkerInit)
-	buf.WriteString(ib.verb)
-	buf.WriteString(" INTO ")
-	buf.WriteString(ib.table)
+
+	if len(ib.table) > 0 {
+		buf.WriteLeadingString(ib.verb)
+		buf.WriteString(" INTO ")
+		buf.WriteString(ib.table)
+	}
+
 	ib.injection.WriteTo(buf, insertMarkerAfterInsertInto)
 
 	if len(ib.cols) > 0 {
-		buf.WriteString(" (")
+		buf.WriteLeadingString("(")
 		buf.WriteString(strings.Join(ib.cols, ", "))
 		buf.WriteString(")")
 
 		ib.injection.WriteTo(buf, insertMarkerAfterCols)
 	}
 
-	buf.WriteString(" VALUES ")
-	values := make([]string, 0, len(ib.values))
+	if len(ib.values) > 0 {
+		buf.WriteLeadingString("VALUES ")
+		values := make([]string, 0, len(ib.values))
 
-	for _, v := range ib.values {
-		values = append(values, fmt.Sprintf("(%v)", strings.Join(v, ", ")))
+		for _, v := range ib.values {
+			values = append(values, fmt.Sprintf("(%v)", strings.Join(v, ", ")))
+		}
+
+		buf.WriteString(strings.Join(values, ", "))
 	}
 
-	buf.WriteString(strings.Join(values, ", "))
 	ib.injection.WriteTo(buf, insertMarkerAfterValues)
 
 	return ib.args.CompileWithFlavor(buf.String(), flavor, initialArg...)

select.go

@@ -234,35 +234,40 @@ func (sb *SelectBuilder) Build() (sql string, args []interface{}) {
 // BuildWithFlavor returns compiled SELECT string and args with flavor and initial args.
 // They can be used in `DB#Query` of package `database/sql` directly.
 func (sb *SelectBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{}) (sql string, args []interface{}) {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	sb.injection.WriteTo(buf, selectMarkerInit)
-	buf.WriteString("SELECT ")
 
-	if sb.distinct {
-		buf.WriteString("DISTINCT ")
+	if len(sb.selectCols) > 0 {
+		buf.WriteLeadingString("SELECT ")
+
+		if sb.distinct {
+			buf.WriteString("DISTINCT ")
+		}
+
+		buf.WriteString(strings.Join(sb.selectCols, ", "))
 	}
 
-	buf.WriteString(strings.Join(sb.selectCols, ", "))
 	sb.injection.WriteTo(buf, selectMarkerAfterSelect)
 
-	buf.WriteString(" FROM ")
-	buf.WriteString(strings.Join(sb.tables, ", "))
+	if len(sb.tables) > 0 {
+		buf.WriteLeadingString("FROM ")
+		buf.WriteString(strings.Join(sb.tables, ", "))
+	}
+
 	sb.injection.WriteTo(buf, selectMarkerAfterFrom)
 
 	for i := range sb.joinTables {
 		if option := sb.joinOptions[i]; option != "" {
-			buf.WriteRune(' ')
-			buf.WriteString(string(option))
+			buf.WriteLeadingString(string(option))
 		}
 
-		buf.WriteString(" JOIN ")
+		buf.WriteLeadingString("JOIN ")
 		buf.WriteString(sb.joinTables[i])
 
 		if exprs := sb.joinExprs[i]; len(exprs) > 0 {
 			buf.WriteString(" ON ")
 			buf.WriteString(strings.Join(sb.joinExprs[i], " AND "))
 		}
-
 	}
 
 	if len(sb.joinTables) > 0 {
@@ -270,14 +275,14 @@ func (sb *SelectBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{
 	}
 
 	if len(sb.whereExprs) > 0 {
-		buf.WriteString(" WHERE ")
+		buf.WriteLeadingString("WHERE ")
 		buf.WriteString(strings.Join(sb.whereExprs, " AND "))
 
 		sb.injection.WriteTo(buf, selectMarkerAfterWhere)
 	}
 
 	if len(sb.groupByCols) > 0 {
-		buf.WriteString(" GROUP BY ")
+		buf.WriteLeadingString("GROUP BY ")
 		buf.WriteString(strings.Join(sb.groupByCols, ", "))
 
 		if len(sb.havingExprs) > 0 {
@@ -289,7 +294,7 @@ func (sb *SelectBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{
 	}
 
 	if len(sb.orderByCols) > 0 {
-		buf.WriteString(" ORDER BY ")
+		buf.WriteLeadingString("ORDER BY ")
 		buf.WriteString(strings.Join(sb.orderByCols, ", "))
 
 		if sb.order != "" {
@@ -303,27 +308,27 @@ func (sb *SelectBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{
 	switch flavor {
 	case MySQL, SQLite, ClickHouse:
 		if sb.limit >= 0 {
-			buf.WriteString(" LIMIT ")
+			buf.WriteLeadingString("LIMIT ")
 			buf.WriteString(strconv.Itoa(sb.limit))
 
 			if sb.offset >= 0 {
-				buf.WriteString(" OFFSET ")
+				buf.WriteLeadingString("OFFSET ")
 				buf.WriteString(strconv.Itoa(sb.offset))
 			}
 		}
 	case CQL:
 		if sb.limit >= 0 {
-			buf.WriteString(" LIMIT ")
+			buf.WriteLeadingString("LIMIT ")
 			buf.WriteString(strconv.Itoa(sb.limit))
 		}
 	case PostgreSQL, Presto:
 		if sb.limit >= 0 {
-			buf.WriteString(" LIMIT ")
+			buf.WriteLeadingString("LIMIT ")
 			buf.WriteString(strconv.Itoa(sb.limit))
 		}
 
 		if sb.offset >= 0 {
-			buf.WriteString(" OFFSET ")
+			buf.WriteLeadingString("OFFSET ")
 			buf.WriteString(strconv.Itoa(sb.offset))
 		}
 
@@ -331,21 +336,21 @@ func (sb *SelectBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{
 		// If ORDER BY is not set, sort column #1 by default.
 		// It's required to make OFFSET...FETCH work.
 		if len(sb.orderByCols) == 0 && (sb.limit >= 0 || sb.offset >= 0) {
-			buf.WriteString(" ORDER BY 1")
+			buf.WriteLeadingString("ORDER BY 1")
 		}
 
 		if sb.offset >= 0 {
-			buf.WriteString(" OFFSET ")
+			buf.WriteLeadingString("OFFSET ")
 			buf.WriteString(strconv.Itoa(sb.offset))
 			buf.WriteString(" ROWS")
 		}
 
 		if sb.limit >= 0 {
 			if sb.offset < 0 {
-				buf.WriteString(" OFFSET 0 ROWS")
+				buf.WriteLeadingString("OFFSET 0 ROWS")
 			}
 
-			buf.WriteString(" FETCH NEXT ")
+			buf.WriteLeadingString("FETCH NEXT ")
 			buf.WriteString(strconv.Itoa(sb.limit))
 			buf.WriteString(" ROWS ONLY")
 		}
@@ -356,7 +361,7 @@ func (sb *SelectBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{
 	}
 
 	if sb.forWhat != "" {
-		buf.WriteString(" FOR ")
+		buf.WriteLeadingString("FOR ")
 		buf.WriteString(sb.forWhat)
 
 		sb.injection.WriteTo(buf, selectMarkerAfterFor)

select_test.go

@@ -275,3 +275,21 @@ func ExampleSelectBuilder_SQL() {
 	// Output:
 	// /* before */ SELECT u.id, u.name, c.type, p.nickname /* after select */ FROM user u /* after from */ JOIN contract c ON u.id = c.user_id RIGHT OUTER JOIN person p ON u.id = p.user_id /* after join */ WHERE u.modified_at > u.created_at /* after where */ ORDER BY id /* after order by */ LIMIT 10 /* after limit */ FOR SHARE /* after for */
 }
+
+// Example for issue #115.
+func ExampleSelectBuilder_customSELECT() {
+	sb := NewSelectBuilder()
+
+	// Set a custom SELECT clause.
+	sb.SQL("SELECT id, name FROM user").Where(
+		sb.In("id", 1, 2, 3),
+	)
+
+	s, args := sb.Build()
+	fmt.Println(s)
+	fmt.Println(args)
+
+	// Output:
+	// SELECT id, name FROM user WHERE id IN (?, ?, ?)
+	// [1 2 3]
+}

stringbuilder.go

@@ -0,0 +1,51 @@
+// Copyright 2023 Huan Du. All rights reserved.
+// Licensed under the MIT license that can be found in the LICENSE file.
+
+package sqlbuilder
+
+import (
+	"io"
+	"strings"
+)
+
+type stringBuilder struct {
+	builder *strings.Builder
+}
+
+var _ io.Writer = new(stringBuilder)
+
+func newStringBuilder() *stringBuilder {
+	return &stringBuilder{
+		builder: &strings.Builder{},
+	}
+}
+
+// WriteLeadingString writes s to internal buffer.
+// If it's not the first time to write the string, a blank (" ") will be written before s.
+func (sb *stringBuilder) WriteLeadingString(s string) {
+	if sb.builder.Len() > 0 {
+		sb.builder.WriteString(" ")
+	}
+
+	sb.builder.WriteString(s)
+}
+
+func (sb *stringBuilder) WriteString(s string) {
+	sb.builder.WriteString(s)
+}
+
+func (sb *stringBuilder) WriteRune(r rune) {
+	sb.builder.WriteRune(r)
+}
+
+func (sb *stringBuilder) Write(data []byte) (int, error) {
+	return sb.builder.Write(data)
+}
+
+func (sb *stringBuilder) String() string {
+	return sb.builder.String()
+}
+
+func (sb *stringBuilder) Reset() {
+	sb.builder.Reset()
+}

struct.go

@@ -9,7 +9,6 @@ import (
 	"reflect"
 	"regexp"
 	"sort"
-	"strings"
 )
 
 var (
@@ -305,7 +304,7 @@ func (s *Struct) selectFromWithTags(table string, with, without []string) (sb *S
 		return
 	}
 
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	cols := make([]string, 0, len(tagged.ForRead))
 
 	for _, sf := range tagged.ForRead {

union.go

@@ -129,20 +129,18 @@ func (ub *UnionBuilder) Build() (sql string, args []interface{}) {
 // BuildWithFlavor returns compiled SELECT string and args with flavor and initial args.
 // They can be used in `DB#Query` of package `database/sql` directly.
 func (ub *UnionBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{}) (sql string, args []interface{}) {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	ub.injection.WriteTo(buf, unionMarkerInit)
 
 	if len(ub.builders) > 0 {
 		needParen := flavor != SQLite
 
 		if needParen {
-			buf.WriteRune('(')
-		}
-
-		buf.WriteString(ub.Var(ub.builders[0]))
-
-		if needParen {
+			buf.WriteLeadingString("(")
+			buf.WriteString(ub.Var(ub.builders[0]))
 			buf.WriteRune(')')
+		} else {
+			buf.WriteLeadingString(ub.Var(ub.builders[0]))
 		}
 
 		for _, b := range ub.builders[1:] {
@@ -163,7 +161,7 @@ func (ub *UnionBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{}
 	ub.injection.WriteTo(buf, unionMarkerAfterUnion)
 
 	if len(ub.orderByCols) > 0 {
-		buf.WriteString(" ORDER BY ")
+		buf.WriteLeadingString("ORDER BY ")
 		buf.WriteString(strings.Join(ub.orderByCols, ", "))
 
 		if ub.order != "" {
@@ -175,14 +173,14 @@ func (ub *UnionBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{}
 	}
 
 	if ub.limit >= 0 {
-		buf.WriteString(" LIMIT ")
+		buf.WriteLeadingString("LIMIT ")
 		buf.WriteString(strconv.Itoa(ub.limit))
 
 	}
 
 	if MySQL == flavor && ub.limit >= 0 || PostgreSQL == flavor {
 		if ub.offset >= 0 {
-			buf.WriteString(" OFFSET ")
+			buf.WriteLeadingString("OFFSET ")
 			buf.WriteString(strconv.Itoa(ub.offset))
 		}
 	}

update.go

@@ -171,36 +171,42 @@ func (ub *UpdateBuilder) Build() (sql string, args []interface{}) {
 // BuildWithFlavor returns compiled UPDATE string and args with flavor and initial args.
 // They can be used in `DB#Query` of package `database/sql` directly.
 func (ub *UpdateBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{}) (sql string, args []interface{}) {
-	buf := &strings.Builder{}
+	buf := newStringBuilder()
 	ub.injection.WriteTo(buf, updateMarkerInit)
-	buf.WriteString("UPDATE ")
-	buf.WriteString(ub.table)
+
+	if len(ub.table) > 0 {
+		buf.WriteLeadingString("UPDATE ")
+		buf.WriteString(ub.table)
+	}
+
 	ub.injection.WriteTo(buf, updateMarkerAfterUpdate)
 
-	buf.WriteString(" SET ")
-	buf.WriteString(strings.Join(ub.assignments, ", "))
+	if len(ub.assignments) > 0 {
+		buf.WriteLeadingString("SET ")
+		buf.WriteString(strings.Join(ub.assignments, ", "))
+	}
+
 	ub.injection.WriteTo(buf, updateMarkerAfterSet)
 
 	if len(ub.whereExprs) > 0 {
-		buf.WriteString(" WHERE ")
+		buf.WriteLeadingString("WHERE ")
 		buf.WriteString(strings.Join(ub.whereExprs, " AND "))
 		ub.injection.WriteTo(buf, updateMarkerAfterWhere)
 	}
 
 	if len(ub.orderByCols) > 0 {
-		buf.WriteString(" ORDER BY ")
+		buf.WriteLeadingString("ORDER BY ")
 		buf.WriteString(strings.Join(ub.orderByCols, ", "))
 
 		if ub.order != "" {
-			buf.WriteRune(' ')
-			buf.WriteString(ub.order)
+			buf.WriteLeadingString(ub.order)
 		}
 
 		ub.injection.WriteTo(buf, updateMarkerAfterOrderBy)
 	}
 
 	if ub.limit >= 0 {
-		buf.WriteString(" LIMIT ")
+		buf.WriteLeadingString("LIMIT ")
 		buf.WriteString(strconv.Itoa(ub.limit))
 
 		ub.injection.WriteTo(buf, updateMarkerAfterLimit)