Mybatis动态SQL

<if>：判断语句，用于单条件分支判断

<choose>（<when>、<otherwise>）：相当于 Java 中的 switch...case...default语句，应用于多条件分支判断

<where>、<trim>、<set>：辅助元素，用于处理一些 SQL 拼装、特殊字符问题

<foreach>：循环语句，常用语 in 语句等列举条件中

<bind>：从 OGNL 表达式中创建一个变量，并将其绑定到上下文，常用于模糊查询 的sql中

if标签

<if>标签，如果test中的条件成立，则执行<if>标签中的语句，用于单条件分支判断

<select id="findUserByIf" parameterType="com.chenpeng.mybatismt.bean.User" resultType="com.chenpeng.mybatismt.bean.User">
        select * from user
        where 1 = 1
        <if test="username!=null and username!=''">
            and username like concat('%',#{username},'%')
        </if>
        <if test="job!=null and job!=''">
            and job = #{job}
        </if>
</select>

测试传入user对象

@Test
    public void test1(){
        User user = new User();
        user.setUsername("chenyu");
        user.setJob("程序猿1");
        List<User> users = userMapper.findUserByIf(user);
        System.out.println(users);
    }

sql语句拼接及结果如下

Preparing: select * from user where 1 = 1 and username like concat('%',?,'%') and job = ? 
Parameters: chenyu(String), 程序猿1(String)
[User{id=1, username='chenyu', job='程序猿1'}]

只传入user对象的username

sql语句拼接及结果如下

Preparing: select * from user where 1 = 1 and username like concat('%',?,'%') 
Parameters: chenyu(String)
[User{id=1, username='chenyu', job='程序猿1'}]

不传入user对象

sql语句拼接及结果如下

Preparing: select * from user where 1 = 1 
Parameters: 
[User{id=1, username='chenyu', job='程序猿1'}, User{id=2, username='cy', job='程序猿2'}]

choose标签

<choose>标签，按顺序判断<when>中test的条件是否成立，如果有一个成立，则结束判断，如果 所有<when>中test的条件都不成立，则执行<otherwise>中的语句，用于多条件分支判断

<select id="findUserByChoose" parameterType="com.chenpeng.mybatismt.bean.User" resultType="com.chenpeng.mybatismt.bean.User">
        select * from user
        where 1 = 1
        <choose>
            <when test="username!=null and username!=''">
                and username like concat('%',#{username},'%')
            </when>
            <when test="job!=null and job!=''">
                and job = #{job}
            </when>
            <otherwise>

            </otherwise>
        </choose>
</select>

测试传入user对象

@Test
    public void test2(){
        User user = new User();
        user.setUsername("chenyu");
        user.setJob("程序猿1");
        List<User> users = userMapper.findUserByChoose(user);
        System.out.println(users);
    }

sql语句拼接及结果如下

Preparing: select * from user where 1 = 1 and username like concat('%',?,'%') 
Parameters: chenyu(String)
[User{id=1, username='chenyu', job='程序猿1'}]

where标签

<where>标签，上面使用<if>或者<choose>标签时，都需要加上where 1=1用来防止后面判断的条件不成立时sql语句报错，使用<where>标签，如果条件成立则加上where，如果条件不成立则去掉where

<select id="findUserByWhere" parameterType="com.chenpeng.mybatismt.bean.User" resultType="com.chenpeng.mybatismt.bean.User">
        select * from user
        <where>
            <if test="username!=null and username!=''">
                and username like concat('%',#{username},'%')
            </if>
            <if test="job!=null and job!=''">
                and job = #{job}
            </if>
        </where>
</select>

测试传入user对象

@Test
    public void test3(){
        User user = new User();
        user.setUsername("chenyu");
        user.setJob("程序猿1");
        List<User> users = userMapper.findUserByWhere(user);
        System.out.println(users);
    }

sql语句拼接及结果如下

Preparing: select * from user WHERE username like concat('%',?,'%') and job = ? 
Parameters: chenyu(String), 程序猿1(String)
[User{id=1, username='chenyu', job='程序猿1'}]

此时加上了where

不传入user对象

sql语句拼接及结果如下

Preparing: select * from user 
Parameters: 
[User{id=1, username='chenyu', job='程序猿1'}, User{id=2, username='cy', job='程序猿2'}]

此时去掉了where

trim标签

<trim>标签同样可以实现<where>标签的功能，但是更加灵活，可以设置前缀和后缀以及覆盖

<select id="findUserByTrim" parameterType="com.chenpeng.mybatismt.bean.User" resultType="com.chenpeng.mybatismt.bean.User">
        select * from user
        <trim prefix="where" prefixOverrides="and"><!--prefixOverrides="and"覆盖username前的and-->
            <if test="username!=null and username!=''">
                and username like concat('%',#{username},'%')
            </if>
            <if test="job!=null and job!=''">
                and job = #{job}
            </if>
        </trim>
</select>

prefixOverrides还可以设置多个，如

<trim prefix="where" prefixOverrides="and|or">

set标签

<set>标签一般用于update，防止当传入对象的某个值为空时，对应表的值也被修改为空

根据id更新user

<update id="updateUserBySet" parameterType="com.chenpeng.mybatismt.bean.User">
        update user set username=#{username},job=#{job} where id=#{id}
</update>

如果不传入user的job

@Test
    public void test5(){
        User user = new User();
        user.setUsername("chenyu11");
        user.setId(1);
        int count = userMapper.updateUserBySet(user);
        System.out.println(count);
    }

运行后查看表，job变为null

使用<set>标签

<update id="updateUserBySet" parameterType="com.chenpeng.mybatismt.bean.User">
        update user
        <set>
            <if test="username!=null and username!=''">
                username=#{username},
            </if>
            <if test="job!=null and job!=''">
                job=#{job},
            </if>
        </set>
        where id=#{id}
</update>

重新运行后查看表，job没有变为null

也可以使用<trim>标签来实现<set>标签的功能

<update id="updateUserByTrim" parameterType="com.chenpeng.mybatismt.bean.User">
        update user
        <trim prefix="set" suffixOverrides=","><!--后缀覆盖-->
            <if test="username!=null and username!=''">
                username=#{username},
            </if>
            <if test="job!=null and job!=''">
                job=#{job},
            </if>
        </trim>
        where id=#{id}
</update>

foreach标签

<foreach>标签有遍历的功能，可以用来进行批量操作

<select id="findUserByIdsForeach" parameterType="list"
            resultType="com.chenpeng.mybatismt.bean.User">
        select * from user
        where id in
        <foreach collection="list" item="id" open="(" close=")" separator=",">
            #{id}
        </foreach>
</select>

其中属性：

• collection：传入的集合，可以填collection或者list
• item：每一项的名称
• open：以什么开始
• close：以什么结束
• separator：分隔符

测试传入一个id集合

@Test
    public void test7(){
        List<Integer> ids = new ArrayList<>();
        ids.add(1);
        ids.add(2);
        List<User> users = userMapper.findUserByIdsForeach(ids);
        System.out.println(users);
    }

sql语句拼接及结果如下

Preparing: select * from user where id in ( ? , ? ) 
Parameters: 1(Integer), 2(Integer)
[User{id=1, username='chenyu111', job='程序猿111'}, User{id=2, username='cy', job='程序猿2'}]

如果传入的id集合为空，则需要配合<where>和<if>标签使用

<select id="findUserByIdsForeach" parameterType="list"
            resultType="com.chenpeng.mybatismt.bean.User">
        select * from user
        <where>
            <if test="list!=null and list.size()>0">
                id in
                <foreach collection="collection" item="id" open="(" close=")" separator=",">
                    #{id}
                </foreach>
            </if>
        </where>
</select>

批量插入

<insert id="insertUserByUserListForeach" parameterType="list">
        insert into user(username,job) values
        <foreach collection="list" item="user" separator=",">
            (#{user.username},#{user.job})
        </foreach>
</insert>

测试运行

@Test
    public void test8(){
        List<User> userList = new ArrayList<>();
        userList.add(new User("cheny","程序猿2"));
        userList.add(new User("cyu","程序猿3"));
        int count = userMapper.insertUserByUserListForeach(userList);
        System.out.println(count);
    }

查看表，多了两条数据

批量删除

<delete id="deleteUserByIdsForeach" parameterType="list">
        delete from user where id in
        <foreach collection="list" item="id" open="(" close=")" separator=",">
            #{id}
        </foreach>
</delete>

测试运行

@Test
    public void test9(){
        List<Integer> ids = new ArrayList<>();
        ids.add(1);
        ids.add(2);
        ids.add(3);
        int count = userMapper.deleteUserByIdsForeach(ids);
        System.out.println(count);
    }

bind标签

使用 bind 拼接字符串不仅可以避免因更换数据库而修改 SQL，也能预防 SQL 注入

例如查询时进行模糊查询，Mysql使用concat拼接，而Oracle使用||拼接，如果更换数据库，则sql要重写，而bind是通用的

<select id="findUserByBind" parameterType="com.chenpeng.mybatismt.bean.User"
            resultType="com.chenpeng.mybatismt.bean.User">
        <bind name="usernamePattern" value="'%',#{username},'%'"/>
        select * from user
        where 1 = 1
        <if test="username!=null and username!=''">
            and username like #{usernamePattern}
        </if>
        <if test="job!=null and job!=''">
            and job = #{job}
        </if>
</select>

测试运行

@Test
    public void test10(){
        User user = new User();
        user.setUsername("cyu");
        user.setJob("程序猿3");
        List<User> users = userMapper.findUserByBind(user);
        System.out.println(users);
    }

sql语句拼接及结果如下

Preparing: select * from user where 1 = 1 and username like ? and job = ? 
Parameters: %(String), 程序猿3(String)
[User{id=4, username='cyu', job='程序猿3'}]