引言
动态SQL：Dynamic SQL。
本节我们来通过 MyBatis 的官方文档进行学习。
Description（描述）
官方描述如下：
One of the most powerful features of MyBatis has always been its Dynamic SQL capabilities. If you have any experience with JDBC or any similar framework, you understand how painful it is to conditionally concatenate strings of SQL together, making sure not to forget spaces or to omit a comma at the end of a list of columns. Dynamic SQL can be downright painful to deal with.
While working with Dynamic SQL will never be a party, MyBatis certainly improves the situation with a powerful Dynamic SQL language that can be used within any mapped SQL statement.
The Dynamic SQL elements should be familiar to anyone who has used JSTL or any similar XML based text processors. In previous versions of MyBatis, there were a lot of elements to know and understand. MyBatis 3 greatly improves upon this, and now there are less than half of those elements to work with. MyBatis employs powerful OGNL based expressions to eliminate most of the other elements:
● if ● choose (when, otherwise) ● trim (where, set) ● foreach
My View
上面这段话，总结起来就三点：
-
MyBatis 在动态SQL这块很强大。
-
你需要会 OGNL 表达式
-
if/choose/trim/foreach
OGNL
用法看这里：OGNL指南
另外，看一下转义：
| 字符 | 十进制 | 转义字符 |
|---|---|---|
| " | " | " |
| & | & | & |
| < | < | < |
| > | > | > |
if/where/trim/choose/foreach
- if 表示 判断
<if test="title != null">
AND title like #{title}
</if>
- where
我们总会在if标签里写AND，那么当第一个不存在时就会出问题： sql: ...WHERE AND ...
因此 where 是解决这个问题的
<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>
</where>
- trim
有人会说了，既然写在前面有问题，那可不可以写在后面呢？当然可以。
来看些 trim 的用法：
<trim prefix="WHERE" prefixOverrides="AND |OR ">
...
</trim>
-----
<trim prefix="SET" suffixOverrides=",">
...
</trim>
总结一下： prefix:前面加什么 prefixOverrides:前面忽略掉什么 suffix:后缀 suffixOverrides:后面忽略掉什么
- choose
<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>
- foreach
遍历一个集合。
WHERE ID in
<foreach item="item" index="index" collection="list"
open="(" separator="," close=")">
#{item}
</foreach>
后记
今天先到这里，后面有感悟，再来补充。
测试代码：DynamicSQL-Demo
需要说明的是：为了保持测试风格，有些返回值类型不正确，导致测试报错，但这并不影响着一节的学习，因为你能够看到sql和结果。