select作用：選取指定的數(shù)據(jù)庫(kù)字段，避免查詢?nèi)哂嗟淖侄巍?/p>

下面簡(jiǎn)單敘述一下select的用法；

1、查詢除了指定字段以外的所有字段

方法：QueryWrapper<T> select(Class<T> entityClass, Predicate<TableFieldInfo> predicate)

    @GetMapping(value = "/test")
    public List<Animal> test(Animal arg) {
        QueryWrapper<Animal> wrapper = Wrappers.query(arg)
                .select(Animal.class, item -> ! item.getColumn().equals("age"));
        return animalService.list(wrapper);
    }

控制臺(tái)打印的sql語(yǔ)句如下：

SELECT
	id,
	create_time,
	NAME,
	update_time,
	del_flag,
	hobby 
FROM
	animal 
WHERE
	del_flag =0

2、查詢指定字段

方法：QueryWrapper<T> select(String... columns)

    @GetMapping(value = "/test")
    public List<Animal> test(Animal arg) {
        QueryWrapper<Animal> wrapper = Wrappers.query(arg)
                .select("id", "name", "hobby");
        return animalService.list(wrapper);
    }

控制臺(tái)打印的sql語(yǔ)句如下：

SELECT id,name,hobby FROM animal WHERE del_flag=0

3、結(jié)合聚合函數(shù)查詢

方法：QueryWrapper<T> select(String... columns)

查詢各個(gè)部門員工的平均年齡，然后按照部門id降序排序?

    @GetMapping(value = "/test")
    public List<Animal> test(Animal arg) {
        QueryWrapper<Animal> wrapper = Wrappers.query(arg)
                .select("dept_id", "AVG(age) AS avgAge")
                .groupBy("dept_id")
                .orderByDesc("dept_id");
        return animalService.list(wrapper);
    }

?控制臺(tái)打印的sql語(yǔ)句如下：

SELECT
	dept_id,
	AVG( age ) AS avgAge 
FROM
	animal 
WHERE
	del_flag = 0 
GROUP BY
	dept_id 
ORDER BY
	dept_id DESC