动态SQl

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.abc.dao.IStudentDao">
    
    <select id="selectStudentsByIf" resultType="Student">
        select id,name,age,score from student where 1=1
        
        <if test="name != null and name != ''">
            and name like '%' #{name} '%'
        </if>
        <if test="age > 0">
            and age &lt; #{age}
        </if>
        
    </select>
    
    <select id="selectStudentsByWhere" resultType="Student">
        select id,name,age,score from student
        <where>
            <if test="name != null and name != ''">
                and name like '%' #{name} '%'
            </if>
            <if test="age > 0">
                and age &lt; #{age}
            </if>
        </where>
    </select>
    
    <!-- 若查询条件中有name，无论有条件中有没有age，都只按照name查询；
    只有当查询条件中只包含age时，才按照age查询；若没有任何的查询条件，则
    不进行查询
     -->
    <select id="selectStudentsByChoose" resultType="Student">
        select id,name,age,score from student
        
        <where>
            <choose>
                <when test="name != null and name != ''">
                    and name like '%' #{name} '%'
                </when>
                <when test="age > 0">
                    and age &lt; #{age}
                </when>
                <otherwise>
                    and 1!=1
                </otherwise>
            </choose>
        </where>
        
    </select>
    
    <select id="selectStudentsByForeachArray" resultType="Student">
        <!-- select * from student where id in (1,5,7) -->
        select id,name,age,score from student
        <if test="array != null and array.length > 0">
             where id in 
            <foreach collection="array" item="myid" open="(" close=")" separator=",">
                #{myid}
            </foreach>
        </if>
    </select>
    
    <select id="selectStudentsByForeachList" resultType="Student">
        <!-- select * from student where id in (1,5,7) -->
        select id,name,age,score from student
        <if test="list != null and list.size > 0">
             where id in 
            <foreach collection="list" item="myid" open="(" close=")" separator=",">
                #{myid}
            </foreach>
        </if>
    </select>
    
    <select id="selectStudentsByForeachList2" resultType="Student">
        <!-- select * from student where id in (1,5,7) -->
        select id,name,age,score from student
        <if test="list != null and list.size > 0">
             where id in 
            <foreach collection="list" item="mystu" open="(" close=")" separator=",">
                #{mystu.id}
            </foreach>
        </if>
    </select>
    
    <select id="selectStudentsByFragment" resultType="Student">
        <!-- select * from student where id in (1,5,7) -->
        select <include refid="fieldNames"/> from student
        <if test="list != null and list.size > 0">
             where id in 
            <foreach collection="list" item="mystu" open="(" close=")" separator=",">
                #{mystu.id}
            </foreach>
        </if>
    </select>
    
    <!-- 定义SQL片断 -->
    <sql id="fieldNames">
        id,name,age,score
    </sql>
    
</mapper>