初识Mybatis(六)之动态sql

Mybatis的标签

在写sql语句中,有两个连接条件如果一个没有值,则查询结果会为空

select * from user where username = ” 唐宝宝” and adderss = “林大”

如果在username传入的参数为null,或者每一传递,则查询结果为空

可以用if来判断,得出结果

<if test="username != null and username!=''">
username  = #{username }
</if>
<if test="adderss != null and adderss !=''">
and adderss= #{adderss}
</if>

where

<!-- where标签可以自动添加where，同时处理sql语句中第一个and关键字 -->
<where>
<if test="username != null and username!=''">
username  = #{username }
</if>
<if test="adderss != null and adderss !=''">
and adderss= #{adderss}
</if>
</where>

sql片段:

在书写sql语句时候,会有一些重复的sql片段,比如,select * from user

Mybatis中可以使将这些相同的片段组合起来,罅隙调用直接用相依的标签就可以

<!-- sql片段 -->

<sql id="selector">
select * from user
</sql>

<!--调用sql片段-->
<select id="findUserByIdsArray" resultType="User" parameterType="QueryVO">
<include refid="selector"></include>
</select>

foreach标签:

在查询的时候,有时候要查询某几个特定的值,普通sql要这样写:

select * from user wh
4000
ere user_id in(1,2,3,4);

但是要用之前的办法无法达到动态修改,要想在程序中动态修改,Mybatis为我们带来了Foreach标签

<!-- foreach查询多个ID -->

<!-- 包装集合 -->
<select id="findUserByIds" resultType="User" parameterType="QueryVo">
<include refid="selector"></include>
<where>
id in
<foreach collection="idlist" item="id" separator="," open="("
close=")">
#{id}
</foreach>
</where>
</select>

<!-- 包装数组 -->
<select id="findUserByIdsArray" resultType="User" parameterType="QueryVO">
<include refid="selector"></include>
<where>
id in
<foreach collection="ids" item="id" separator="," open="("
close=")">
#{id}
</foreach>
</where>
</select>

<!-- 普通集合 -->
<select id="selectUserByids" resultType="user" parameterType="QueryVo">
<include refid="selector">
</include>
<where>
id in
<foreach collection="list" item="id" separator="," open="("
close=")">
#{id}
</foreach>
</where>
</select>
<!--普通数组 -->
<select id="selectUserByIdsArray" resultType="User">
select * from user where id in
<foreach collection="array" item="id" separator="," open="("
close=")">
#{id}
</foreach>
</select>

<!-- ofType:泛型的类型 -->
<resultMap type="User" id="userOrders">
<id column="uid" property="id"/>
<result  column="username" property="username"/>
<collection property="orders" ofType="Orders">
<id column="oid" property="id"/>
<result  column="number" property="number"/>
<result  column="user_id" property="userId"/>
</collection>
</resultMap>