通常在通过组合若干条件进行数据库操作时，需要根据条件组装 sql ，可以使用 <if> 标签。

<select id="findByCondition" parameterType="com.rolex.microlabs.model.User"
            resultType="com.rolex.microlabs.model.User">
    select id, name, age, gender, skill from t_user
    <where>
        <if test="name != null">and name=#{name}</if>
        <if test="age != null">and age=#{age}</if>
        <if test="gender != null">and gender=#{gender}</if>
    </where>
</select>

还有一种类似 switch 功能的标签。

<select id="findByAnyCondition" parameterType="com.rolex.microlabs.model.User"
            resultType="com.rolex.microlabs.model.User">
    select id, name, age, gender, skill from t_user
    <where>
        <choose>
            <when test="name != null">and name=#{name}</when>
            <when test="age != null">and age=#{age}</when>
            <when test="gender != null">and gender=#{gender}</when>
        </choose>
    </where>
</select>

更新操作也可以使用 <if> 标签，可用于更新有值的字段。

<update id="update" parameterType="com.rolex.microlabs.model.User">
    update t_user
    <set>
        <if test="name != null">name=#{name},</if>
        <if test="age != null">age=#{age},</if>
        <if test="gender != null">gender=#{gender}</if>
    </set>
    where id=#{id}
</update>

批量操作时可以使用 <foreach> 标签。

<insert id="batchSave">
    insert into t_user
    (name, age, gender, skill)
    values
    <foreach collection="list" item="user" index="index" separator=",">
        (#{user.name}, #{user.age}, #{user.gender}, #{user.skill} )
    </foreach>
</insert>
<select id="batchQuery" resultType="com.rolex.microlabs.model.User">
    select name, age, gender, skill from t_user
    where id in
    <foreach collection="list" item="id" index="index" open="(" close=")" separator=",">
        #{id}
    </foreach>
</select>
<delete id="batchDelete">
    delete from t_user where id in
    <foreach collection="list" item="id" index="index" open="(" close=")" separator=",">
        #{id}
    </foreach>
</delete>
<update id="batchUpdate">
    <foreach collection="list" item="user" index="index" separator=";">
        update t_user
        <set>
            <if test="user.name != null">name=#{user.name},</if>
            <if test="user.age != null">age=#{user.age},</if>
            <if test="user.gender != null">gender=#{user.gender}</if>
        </set>
        where id=#{user.id}
    </foreach>
</update>