023 - group by

GROUP BY语句将具有相同值的行分组到汇总行中

GROUP BY语句通常与聚合函数（COUNT，MAX，MIN，SUM，AVG）一起使用，将结果集分组为一列或多列。

SQL GROUP BY 语法

SELECT column_name(s) FROM table_name WHERE condition GROUP BY column_name(s) ORDER BY column_name(s);

-- 按照title 分组：

select title from employee group by title;

-- 统计按照title分组，每组数据多少：

SELECT COUNT(title) AS "分组统计" FROM employee GROUP BY title;

-- distinct筛选去重与group by分组有相同之处：

SELECT DISTINCT(title) FROM employee;

-- 分组升序排列：

SELECT last_name FROM employee GROUP BY last_name ORDER BY last_name;

-- 分组降序排列：

SELECT last_name FROM employee GROUP BY last_name ORDER BY last_name DESC;

-- 分组查询不同的值的数据：

SELECT COUNT(DISTINCT salary) FROM employee GROUP BY salary;

-- 查询分组数据数：

SELECT salary, COUNT(*)FROM employee GROUP BY salary ORDER BY salary;

-- 按照分组数据升序 / 降序排列：

-- 升序
SELECT salary, COUNT(*)FROM employee GROUP BY salary ORDER BY COUNT(salary);


-- 降序
SELECT salary, COUNT(*)FROM employee GROUP BY salary ORDER BY COUNT(salary) DESC;