MyBatis中如何动态拼接Sql字符串呢？

MyBatis简介

MyBatis拥有非常强大的动态SQL处理能力
下文笔者讲述动态sql的相关简介说明，如下所示:

if

<select id="findUserWithUserName"
parameterType="User" resultType="User">
SELECT * FROM User
WHERE state = 'ACTIVE'
<if test="userName != null">
AND userName like #{userName}
</if>
</select>

当没有传递userName信息时，则不传入此参数

---

choose,when,otherwise

<select id="findUserWithUserName"
parameterType="User" resultType="User">
SELECT * FROM User
WHERE
<choose>
<when test="userName != null">
AND userName like #{userName}
</when>
<when test="lastName != null ">
AND userName like #{lastName}
</when>
<otherwise> 
</otherwise>
</choose>
</select>

set

<update id="updateUserInfoById"
parameterType="User">
update User
<set>
<if test="username != null">username=#{username},</if>
<if test="password != null">password=#{password},</if>
<if test="email != null">email=#{email}</if>
</set>
where id=#{id}
</update>

同上的问题，优化后

<update id="updateUserInfoById"
parameterType="User">
update User
<trim prefix="where" prefixOverrides=",">
<set>
<if test="username != null">username=#{username},</if>
<if test="password != null">password=#{password},</if>
<if test="email != null">email=#{email}</if>
</set>
where id=#{id}
</trim>
</update>