Mybatis学习三（动态sql语句）

动态sql语句主要为以下语句

1、动态SQL:if 语句
2、动态SQL:if+where 语句
3、动态SQL:if+set 语句
4、动态SQL:choose(when,otherwise) 语句
5、动态SQL:trim 语句
6、动态SQL: SQL 片段
7、动态SQL: foreach 语句

之前的几篇学习博客我用mybatis对一张表进行的CRUD操作，但是 SQL 语句都比较简单，如果有比较复杂的 SQL 语句，经常需要拼接，而拼接 SQL稍微不注意就容易出错此时我们可以用mybatis 动态SQL，通过 if, choose, when, otherwise, trim, where, set, foreach等标签来解决这个问题，这几个标签可组合成非常灵活的SQL语句，用起来也是很舒服

<?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">
<!-- namespace:表示名称空间。现在的目的是区分id的. -->
<mapper namespace="com.zhiyou100.zhl.dao.UsersDao">
    <sql id="detail">
        id,name,age,sex,day
    </sql>
    
    <select id="selByWhere" resultType="com.zhiyou100.zhl.bean.Users">
        select * from users
        <where>
            <if test="name!=null">
                and name=#{name}
            </if>
            <if test="sex!=null and sex!=''">
                and sex=#{sex}
            </if>
        </where>
    </select>
    
    <update id="updateWhere">
        update users
        <set>
            <if test="name!=null">
                name=#{name},
            </if>
            <if test="sex!=null">
                sex=#{sex},
            </if>
            <if test="age>0">
                age=#{age},
            </if>
            <if test="day!=null">
                day=#{day}
            </if>
        </set>
        where id=#{id}
    </update>
    
    <select id="selByWhere2" resultType="com.zhiyou100.zhl.bean.Users">
        select
        <include refid="detail"/>
        from users
        <where>
            <choose>
                <when test="name!=null and name!=''">
                    name like concat('%',#{name},'%')
                </when>
                <when test="sex!=null and sex!=''">
                    sex=#{sex}
                </when>
                <otherwise>
                    age>=#{age}
                </otherwise>
            </choose>
        </where>
    </select>
    
    <select id="selByWhere3" resultType="com.zhiyou100.zhl.bean.Users">
        select
        <include refid="detail"/>
        from users
        <trim prefix="where" prefixOverrides="and | or">
            <if test="name!=null and name!=''">
                and name=#{name}
            </if>
            <if test="sex!=null and sex!=''">
                and sex=#{sex}
            </if>
            <if test="age>0">
                and age=#{age}
            </if>
            <if test="day!=null and day!=''">
                and day=#{day}
            </if>
        </trim>
    </select>
    
    <update id="updateWhere2">
        update users
        <trim prefix="set" suffixOverrides=",">
            <if test="name!=null">
                name=#{name},
            </if>
            <if test="sex!=null">
                sex=#{sex},
            </if>
            <if test="age>0">
                age=#{age},
            </if>
            <if test="day!=null">
                day=#{day}
            </if>
        </trim>
        where id=#{id}
    </update>
    
    <delete id="deleteByIds">
        delete from users
        <where>
            <foreach collection="ids" open="id in(" close=")" separator="," item="id">
                #{id}
            </foreach>
        </where>
    </delete>
    
    
    <delete id="deleteByIds2">
        delete from users where id in
        <foreach collection="ids" open="(" close=")" separator="," item="id">
            #{id}
        </foreach>
    </delete>
    
    
</mapper>