mabatis--动态sql

1、mybatis核心，对sql语句进行灵活操作，通过表达式进行判断，对sql进行灵活拼接、组装；

2、使用if判断:

<where>
    <if test="customer!=null">
        <if test="customer.email!=null and customer.email!=''">
            and email like '%${customer.email}%'
        </if>
        <if test="customer.name!=null and customer.name!=''">
            and name like '%${customer.name}%'
        </if>
    </if>
</where>

3、sql片段：可以将sql语句语句中的某一片段提取出来，供其他statement引用：

<!-- 在sql片段中尽量不要使用<where></where>，因为一个statement可以引用多个sql片段，
若在sql片段中写入了<where></where>,则有可能出现sql语句拼接语法的错误 -->
<sql id="find_customer_where">
    <if test="customer!=null">
        <if test="customer.email!=null and customer.email!=''">
            and email like '%${customer.email}%'
        </if>
        <if test="customer.name!=null and customer.name!=''">
            and name like '%${customer.name}%'
        </if>
    </if>
</sql>

<!-- 引用sql片段 -->
<select id="findCustomerList" parameterType="CustomerQueryVo" resultType="Customer">
    select * from customers
    <where>
        <include refid="find_customer_where"></include>
    </where>    
</select>

4、foreach: 

<sql id="find_customer_foreach">
    <if test="ids!=null">
        <!-- select * from customer where (id=1 or id=2 or id=5)-->
        <foreach collection="ids" item="id" open="and (" close=")" separator="or">
        id=#{id}
        </foreach>
        <!-- select * from customer where id in (1, 2, 5) -->
        <!-- <foreach collection="ids" item="id" open="and id in(" close=")" separator=",">
        #{id}
        </foreach> -->
    </if>
</sql>