目前我們在查詢數據的時候,什麼都沒有做默認就是查詢表中所有字段的內容,我們所說的查詢投影即不查詢所有字段,只查詢出指定內容的數據。
具體如何來實現?
@SpringBootTest
class Mybatisplus02DqlApplicationTests {
@Autowired
private UserDao userDao;
@Test
void testGetAll(){
LambdaQueryWrapper<User> lqw = new LambdaQueryWrapper<User>();
lqw.select(User::getId,User::getName,User::getAge);
List<User> userList = userDao.selectList(lqw);
System.out.println(userList);
}
}
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select(...)方法用來設置查詢的字段列,可以設置多個,最終的sql語句爲:
SELECT id,name,age FROM user
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如果使用的不是lambda,就需要手動指定字段
@SpringBootTest class Mybatisplus02DqlApplicationTests { @Autowired private UserDao userDao; @Test void testGetAll(){ QueryWrapper<User> lqw = new QueryWrapper<User>(); lqw.select("id","name","age","tel"); List<User> userList = userDao.selectList(lqw); System.out.println(userList); } }
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最終的sql語句爲:SELECT id,name,age,tel FROM user
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2、聚合查詢
需求:聚合函數查詢,完成count、max、min、avg、sum的使用
count:總記錄數
max:最大值
min:最小值
avg:平均值
sum:求和
@SpringBootTest
class Mybatisplus02DqlApplicationTests {
@Autowired
private UserDao userDao;
@Test
void testGetAll(){
QueryWrapper<User> lqw = new QueryWrapper<User>();
//lqw.select("count(*) as count");
//SELECT count(*) as count FROM user
//lqw.select("max(age) as maxAge");
//SELECT max(age) as maxAge FROM user
//lqw.select("min(age) as minAge");
//SELECT min(age) as minAge FROM user
//lqw.select("sum(age) as sumAge");
//SELECT sum(age) as sumAge FROM user
lqw.select("avg(age) as avgAge");
//SELECT avg(age) as avgAge FROM user
List<Map<String, Object>> userList = userDao.selectMaps(lqw);
System.out.println(userList);
}
}
爲了在做結果封裝的時候能夠更簡單,我們將上面的聚合函數都起了個名稱,方面後期來獲取這些數據
3、分組查詢
需求:分組查詢,完成 group by的查詢使用
@SpringBootTest
class Mybatisplus02DqlApplicationTests {
@Autowired
private UserDao userDao;
@Test
void testGetAll(){
QueryWrapper<User> lqw = new QueryWrapper<User>();
lqw.select("count(*) as count,tel");
lqw.groupBy("tel");
List<Map<String, Object>> list = userDao.selectMaps(lqw);
System.out.println(list);
}
}
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groupBy爲分組,最終的sql語句爲
SELECT count(*) as count,tel FROM user GROUP BY tel
注意:
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聚合與分組查詢,無法使用lambda表達式來完成
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MybatisPlus只是對MyBatis的增強,如果MybatisPlus實現不了,我們可以直接在DAO接口中使用MyBatis的方式實現