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http://study.163.com/course/introduction.htm?courseId=1004329008&utm_campaign=commission&utm_source=400000000155061&utm_medium=share
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https://gitee.com/happyangellxq520/spring-boot
à Spring Boot communication platform
http://412887952-qq-com.iteye.com/blog/2321532
[This blog has a supporting video, the video address: " Spring Boot MyBatis Upgrade - Annotation - Dynamic SQL (if test) - Scheme 1 script " , click the following to read the original text in the official account, and the video explains in more detail]
Origin of need:
Netizen 1 : How to solve the query if it is an indefinite condition? That is , if tags can be used in xml , what about annotations?
Netizen 2 : This dynamic sql is not very convenient to use annotations! There is a good way for bloggers, please refer to it, the reason why the annotation has not been used is that there is no good way for dynamic sql , including when foreach ! For simple queries it's still ok!
The blogger is very relieved to see such positive questions from netizens. If there is a problem, he will study it.
Demand scenario:
When we query a table with id, name, and email, we hope that when the name is not null , it will be used as the query condition, and if the email is not null , it will be used as the query condition.
Implementation process:
( 1 ) name, email and relationship
如果name和email只是并且的关系的话，那么我们会这么写：
@Select("Select * from Demo where name =#{name} and email=#{email} ")
public List<Demo> select3(Demo demo);
其中Demo类如下：
public class Demo {
private int id;
private String name;
private String email;
private Date updateTime;
private SexEnum sexEnum;
//省略getter and setter…
}
表中的数据：
2 王五 [email protected] 3 王五 [email protected] 4 王五2 [email protected] 5 王五6 [email protected]
这时候访问：
http://127.0.0.1:8080/select3?name=王五&[email protected] 能返回数据。
但是现在我们的需求不是这样的了。
（2）if name !=null ,if email != null
现在我们希望的是如果name不为null的话，那么就当做条件，否则就不要当做条件；如果email不为null，那么就当做条件，否则不当做条件。
那么方案一：so eay,只需要在前面加入<script>就可以使用<if test>标签了，代码如下：
@Select("<script> " +
"SELECT * " +
"from Demo " +
" <where> " +
" 1=1" +
" <if test=\"name != null\">and name=#{name}</if> " +
" <if test=\"email != null\"> and email=#{email}</if> " +
" </where> " +
" </script> ")
public List<Demo> select4(Demo demo);
访问1：http://127.0.0.1:8080/select4会返回全部数据，动态SQL是：
SELECT * from Demo WHERE 1=1
访问2：http://127.0.0.1:8080/select4?name=王五 会返回name=王五的数据，动态SQL是：
SELECT * from Demo WHERE 1=1 and name=?
访问3：http://127.0.0.1:8080/select4?name=王五&[email protected]会返回name=王五并且[email protected]的数据，动态SQL是：
SELECT * from Demo WHERE 1=1 and name=? and email=?
是不是很酷，完美解决问题。但是后头看代码这样的代码可读性差，修改的时候也很麻烦，所以呢，是否有更好的方案呢？下篇博客揭晓，广告时刻，大家不要离开。
视频&交流平台：
http://study.163.com/course/introduction.htm?courseId=1004329008
http://412887952-qq-com.iteye.com/blog/2321532