文章目录
MP这样一款强大的持久层框架处理起来复杂的SQL来也是得心应手，效率极高，快快与我一同领略Plus的独特魅力吧
image.png
一.分页处理
1.调用方法传入参数获取返回值
创建IPage分页对象,设置分页参数,1为当前页码，3为每页显示的记录数，执行分页查询并获取其结果
<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@SpringBootTest
class Mybatisplus{
@Autowired
private UserDao userDao;
//分页查询
@Test
void testSelectPage(){
IPage<User> page=new Page<>(1,3);
userDao.selectPage(page,null);
System.out.println("当前页码值："+page.getCurrent());
System.out.println("每页显示数："+page.getSize());
System.out.println("一共多少页："+page.getPages());
System.out.println("一共多少条数据："+page.getTotal());
System.out.println("数据："+page.getRecords());
}
}</pre>
2.设置分页拦截器
将MP提供的分页拦截器配置成Spring管理的bean对象
<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@Configuration
public class MybatisPlusConfig {
@Bean
public MybatisPlusInterceptor mybatisPlusInterceptor(){
//1 创建MybatisPlusInterceptor拦截器对象
MybatisPlusInterceptor mpInterceptor=new MybatisPlusInterceptor();
//2 添加分页拦截器
mpInterceptor.addInnerInterceptor(new PaginationInnerInterceptor());
return mpInterceptor;
}
}</pre>
查询结果如下：
image.png
二.条件查询
2.1通过QueryWrapper对象来执行分页查询
<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@SpringBootTest
class Mybatisplus{
@Autowired
private UserDao userDao;
@Test
void testGetAll(){
QueryWrapper qw = new QueryWrapper();
qw.lt("age",18);
List<User> userList = userDao.selectList(qw);
System.out.println(userList);
}
}</pre>
注：lt（）方法为小于(<) ,对应的SQL为：
<pre class="prettyprint hljs sql" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT id,name,password,age,tel FROM user WHERE (age < ?)</pre>
很容易发现， 以字符串形式输出作为查询条件可能会出现字符串拼写错误 ，针对此种情况，可以进行一下小改进！
2.2在QueryWrapper对象的基础上使用lambda表达式
为了解决以字符串形式作为输出而造成拼写错误的问题， 通过lambda来实现实体与属性对应进行查询，就极大地提高了查询的准确性
<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@SpringBootTest
class Mybatisplus{
@Autowired
private UserDao userDao;
@Test
void testGetAll(){
QueryWrapper<User> qw = new QueryWrapper<User>();
qw.lambda().lt(User::getAge, 10);//添加条件
List<User> userList = userDao.selectList(qw);
System.out.println(userList);
}
}</pre>
与之对应的SQL语句同样也是：
<pre class="prettyprint hljs sql" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT id,name,password,age,tel FROM user WHERE (age < ?)</pre>
注：构建LambdaQueryWrapper的时候泛型不能省
image.png
当不使用泛型时会提示默认的Object类不是函数接口
image.png
而我们的 lambda（）的底层又需要传进去一个实体 ，传进去Object显然不能与后面的查询条件相联系！
此时我们再次编写条件的时候，就不会存在写错名称的情况，但是qw后面多了一层lambda()调用
2.3直接通过LambdaQueryWrapper对象
这也是方式二的另一种写法，原理相同都是利用LambdaQueryWrapper
<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@SpringBootTest
class Mybatisplus{
@Autowired
private UserDao userDao;
@Test
void testGetAll(){
LambdaQueryWrapper<User> lqw = new LambdaQueryWrapper<User>();
lqw.lt(User::getAge, 10);
List<User> userList = userDao.selectList(lqw);
System.out.println(userList);
}
}</pre>
三.多条件查询
对于多条件的情景，MP依然可以简单化解，并且构建多条件的时候，可以支持链式编程
3.1且的情况
场景一：查询数据库表中，年龄在3岁到8岁之间的用户信息
<pre class="prettyprint hljs haskell" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@SpringBootTest
class Mybatisplus{ @Autowired private UserDao userDao; @Test /**
* 多条件查询
*/ void testGetAll04() { //方式四 (常用！) LambdaQueryWrapper<Users> qw4 = new LambdaQueryWrapper<>(); qw4.lt(Users::getAge, 8); //上限 qw4.gt(Users::getAge, 3); //下限 // qw4.lt(Users::getAge, 8).gt(Users::getAge, 3); 链式编程！ List<Users> users = userDao.selectList(qw4); System.out.println(users); } }</pre>
注：gt（），大于(>),最终的SQL语句为
<pre class="prettyprint hljs sql" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT id,name,password,age,tel FROM user WHERE (age < ? AND age > ?)</pre>
也是迅速查出来了结果
image.png
3.2或的情况
场景二：查询数据库表中，年龄小于3或年龄大于8的数据
<pre class="prettyprint hljs haskell" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@SpringBootTest
class Mybatisplus{ @Autowired private UserDao userDao; @Test void testGetAll(){
LambdaQueryWrapper<User> lqw = new LambdaQueryWrapper<User>(); lqw.lt(User::getAge, 3).or().gt(User::getAge, 8); List<User> userList = userDao.selectList(lqw); System.out.println(userList); } }</pre>
这里的or()就相当于sql语句中的 or 关键字,不加默认是 and ，最终的sql语句为:
<pre class="prettyprint hljs sql" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT id,name,password,age,tel FROM user WHERE (age < ? OR age > ?)</pre>
也是顺利的查了出来
image.png
四.null判定
image.png
以TB为例，我们购物时进行条件筛选时， 可以选择单条件，也可以选择多条件 ，如上，我的条件就变成price>3000,price<null，这种情况按照以上介绍的查询的方式就会出现问题，如图：
image.png
显然，这种情况在开发过程中时不被允许的。所以要求我们针对null的情况要解决如下问题：
用户在输入值的时候：
1.如果只输入第一个框，说明要查询大于该价格的商品
2.如果只输入第二个框，说明要查询小于该价格的商品 3.如果两个框都输入了，说明要查询价格在两个范围之间的商品
于是，我们可以
新建一个模型类,让其 继承Brand类，并在其中添加price2属性 ，Brand02 在拥有Brand属性后同时添加了price2属性
<pre class="prettyprint hljs scala" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@Data
public class Brand {
private Long id;
private String name;
private Double price;
}
@Data
public class Brand02 extends Brand {
private Integer price2;
}</pre>
解决了实体的问题，再来解决条件的问题
<pre class="prettyprint hljs groovy" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; word-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@SpringBootTest
class Mybatisplus02{
@Autowired
private BrandDao brandDao;
@Test
void testGetAll(){
BrandQuery bq = new BrandQuery();
LambdaQueryWrapper<Brand> lqw = new LambdaQueryWrapper<Brand>();
lqw.lt(null!=bq.getPrice2(),User::getPrice, bq.getPrice2());
lqw.gt(null!=bq.getPrice(),User::getPrice, bq.getPrice());
List<Brand> brands = brandDao.selectList(lqw);
System.out.println(brands);
}
}</pre>
解读:
如果两个属性不为空，则查询price，price2区间范围内
实现的核心在于lt()、gt()方法，condition为boolean类型上述的 null!=bq.getPrice2() 与之对应， 返回true，则添加条件，返回false则不添加条件，条件的生效与否就是靠的这个设计！
image.png
最后，也是在null的条件下完成了查询：
image.png
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