Mybatis08_动态SQL
if 标签一般和 where 标签搭配使用，where 标签会自动判断是否要删除语句块中的 and 关键字
<select id="findByAccount" parameterType="com.blu.entity.Account">
select * from t_account
<where>
<if test="id!=0">
id = #{id}
</if>
<if test="username!=null">
and username = #{username}
</if>
<if test="password!=null">
and password = #{password}
</if>
<if test="age!=0">
and age = #{age}
</if>
</where>
</select>
choose 标签与 where 标签搭配使用，将按顺序匹配，只要有条件满足则 choose 结束，相当于 or，类似Java 的 switch 语句
<select id="findByAccount" parameterType="com.blu.entity.Account">
select * from t_account
<where>
<choose>
<when test="id!=0">
id = #{id}
</when>
<when test="username!=null">
username = #{username}
</when>
<when test="password!=null">
password = #{password}
</when>
<otherwise>
age = #{age}
</otherwise>
</choose>
</where>
</select>
trim 标签的prefix表示加上前缀where，prefixOverrides 表示去掉第一个关键字and，作用和 if 与 where 的搭配相同
<select id="findByAccount" parameterType="com.blu.entity.Account">
select * from t_account
<trim prefix="where" prefixOverrides="and">
<if test="id!=0">
and id = #{id}
</if>
<if test="username!=null">
and username = #{username}
</if>
<if test="password!=null">
and password = #{password}
</if>
<if test="age!=0">
and age = #{age}
</if>
</trim>
</select>
set 标签用于在更新操作中拼接SQL
<update id="update" parameterType="com.blu.entity.Account">
update t_account
<set>
<if test="username!=null">
username = #{username}
</if>
<if test="password!=null">
password = #{password}
</if>
<if test="age!=0">
age = #{age}
</if>
</set>
where id = #{id}
</update>
foreach 标签
略
