[MyBatis]动态SQL

动态SQL

参考链接:https://mybatis.org/mybatis-3/zh/dynamic-sql.html

概念

动态Sql就是动态的拼接Sql语句，MyBatis使动态Sql变得容易，只需要编写XML

涉及的标签

if

通过if标签可以动态的拼接where语句

<select id="findActiveBlogWithTitleLike"
     resultType="Blog">
  SELECT * FROM BLOG
  WHERE state = ‘ACTIVE’
  <if test="title != null">
    AND title like #{title}
  if>
select>

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG WHERE state = ‘ACTIVE’
  <if test="title != null">
    AND title like #{title}
  if>
  <if test="author != null and author.name != null">
    AND author_name like #{author.name}
  if>
select>

choose(when、otherwise)

如果想从多个条件中选择一个使用可以使用choose标签，类似switch语句

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG WHERE state = ‘ACTIVE’
  <choose>
    <when test="title != null">
      AND title like #{title}
    when>
    <when test="author != null and author.name != null">
      AND author_name like #{author.name}
    when>
    <otherwise>
      AND featured = 1
    otherwise>
  choose>
select>

trim(where、set)

下面代码生成的Sql是不正确的

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG
  WHERE
  <if test="state != null">
    state = #{state}
  if>
  <if test="title != null">
    AND title like #{title}
  if>
  <if test="author != null and author.name != null">
    AND author_name like #{author.name}
  if>
select>

SELECT * FROM BLOG
WHERE

如果只匹配第二个条件生成的SQL也是不对的

SELECT * FROM BLOG
WHERE
AND title like ‘someTitle’

这种情况就可以使用where标签，可以自动的去除"前缀"where并去除多余的"AND"和"OR"

where标签等价于下面的trim标签：

<trim prefix="WHERE" prefixOverrides="AND |OR ">
  ...
trim>

注意空格不可省。

set标签可以动态包含需要更新的列

<update id="updateAuthorIfNecessary">
  update Author
    <set>
      <if test="username != null">username=#{username},if>
      <if test="password != null">password=#{password},if>
      <if test="email != null">email=#{email},if>
      <if test="bio != null">bio=#{bio}if>
    set>
  where id=#{id}
update>

相当于下面的trim标签:

<trim prefix="SET" suffixOverrides=",">
  ...
trim>

所以说可以通过trim标签来自定义行为

foreach

foreach标签用来对集合（可迭代对象）进行遍历，比如构建In语句

<select id="selectPostIn" resultType="domain.blog.Post">
  SELECT *
  FROM POST P
  WHERE ID in
  <foreach item="item" index="index" collection="list"
      open="(" separator="," close=")">
        #{item}
  foreach>
select>

当使用可迭代对象或者数组时，index 是当前迭代的序号，item 的值是本次迭代获取到的元素。当使用 Map 对象（或者 Map.Entry 对象的集合）时，index 是键，item 是值。

注解中使用动态SQL

要在带注解的映射器接口类中使用动态 SQL，可以使用 script 元素。比如:

@Update({""})
void updateAuthorValues(Author author);