DELETE Statement-Toy模板网

这篇具有很好参考价值的文章主要介绍了DELETE Statement。希望对大家有所帮助。如果存在错误或未考虑完全的地方，请大家不吝赐教，您也可以点击"举报违法"按钮提交疑问。

DELETE Statement

DELETE 应该是增删改查里最简单的语句了

语法分析

由于不同类型 db 包括 SQLite3、 MySQL、PostgreSQL 的 DELETE 最简单形态的语法是一样的，本文只以实现最简单的形态为目标，所以，这里只拿 MySQL 举例
MYSQL 的 DELETE 语句也有两种形态:

删除单表的：额外支持了 ORDER BY 和 LIMIT

删除多表的：只支持 WHERE 条件

开源实例

Beego ORM

Beego 的 DELETE API 定义和 UPDATE 一样，如果 cols 没有传，默认是根据主键进行删除。

GORM

DELETE Statement
DELETE 相关方法只有一个，具体实现思路是，删除一条记录时，删除对象需要指定主键，否则会触发批量Delete，例如：

// Email 的 ID 是 `10`
db.Delete(&email)
// DELETE from emails where id = 10;

// 带额外条件的删除
db.Where("name = ?", "jinzhu").Delete(&email)
// DELETE from emails where id = 10 AND name = "jinzhu";

db.Delete(&users, []int{1,2,3})
// DELETE FROM users WHERE id IN (1,2,3);

API 设计

与 Update 类似，需要结构体 Deletor 结构体去实现 QueryBuilder 与 Executor 接口。以及需要包含条件语句的拼接。

type Deleter[T any] struct {
}

func (d *Deleter[T]) Build() (*Query, error) {
	panic("implement me")
}

// From accepts model definition
func (d *Deleter[T]) From(table string) *Deleter[T] {
	panic("implement me ")
}

// Where accepts predicates
func (d *Deleter[T]) Where(predicates ...Predicate) *Deleter[T] {
	panic("implement me")
}

具体实现

Build 方法

// Deleter builds DELETE query
type Deleter[T any] struct {
	builder
	table string
	where []Predicate
}

// Build returns DELETE query
func (d *Deleter[T]) Build() (*Query, error) {
	_, _ = d.sb.WriteString("DELETE FROM ")

	if d.table == "" {
		var t T
		d.sb.WriteByte('`')
		d.sb.WriteString(reflect.TypeOf(t).Name())
		d.sb.WriteByte('`')
	} else {
		d.sb.WriteString(d.table)
	}
	if len(d.where) > 0 {
		d.sb.WriteString(" WHERE ")
		err := d.buildPredicates(d.where)
		if err != nil {
			return nil, err
		}
	}
	d.sb.WriteByte(';')
	return &Query{SQL: d.sb.String(), Args: d.args}, nil
}

// From accepts model definition
func (d *Deleter[T]) From(table string) *Deleter[T] {
	d.table = table
	return d
}

// Where accepts predicates
func (d *Deleter[T]) Where(predicates ...Predicate) *Deleter[T] {
	d.where = predicates
	return d
}

与 Update 一样需要实现一个 Excute方法

func (d *Deleter[T]) Exec(ctx context.Context) Result {
	q, err := d.Build()
	if err != nil {
		return Result{err: err}
	}
	res, err := d.db.db.ExecContext(ctx, q.SQL, q.Args...)
	return Result{err: err, res: res}
}

单元测试

func TestDeleter_Build(t *testing.T) {
	testCases := []struct {
		name      string
		builder   QueryBuilder
		wantErr   error
		wantQuery *Query
	}{
		{
			name:    "no where",
			builder: (&Deleter[TestModel]{}).From("`test_model`"),
			wantQuery: &Query{
				SQL: "DELETE FROM `test_model`;",
			},
		},
		{
			name:    "where",
			builder: (&Deleter[TestModel]{}).Where(C("Id").EQ(16)),
			wantQuery: &Query{
				SQL: "DELETE FROM `TestModel` WHERE `Id` = ?;",
				Args: []any{16},
			},
		},
		{
			name:    "from",
			builder: (&Deleter[TestModel]{}).From("`test_model`").Where(C("Id").EQ(16)),
			wantQuery: &Query{
				SQL: "DELETE FROM `test_model` WHERE `Id` = ?;",
				Args: []any{16},
			},
		},
	}

	for _, tc := range testCases {
		c := tc
		t.Run(c.name, func(t *testing.T) {
			query, err := c.builder.Build()
			assert.Equal(t, c.wantErr, err)
			if err != nil {
				return
			}
			assert.Equal(t, tc.wantQuery, query)
		})
	}
}