柚子酱的Mysql学习笔记（二）

聚集函数

1.AVG函数

SELECT AVG(col_name1) AS avg_col
FROM table_name;

返回特定行的平均值

SELECT AVG(col_name) AS avg_col
FROM table_name
WHERE condition;

2.COUNT()函数
确定表中行的数目或满足特定条件的行的数目,忽略NULL值

SELECT COUNT(*) AS num_col
FROM table_name;

其余函数不再赘述，使用方法相同

3.聚集不同的值
若有些行的值相同，不再重复计算，使用DISTINCT参数

SELECT AVG(DISTINCT col_name) AS avg_col
FROM table_name
WHERE condition

4.组合聚集函数

SELECT COUNT(*) AS num_col,
		MIN(col_name) AS min_col,
		MAX(col_name) AS max_col,
		AVG(col_name) AS avg_col
FROM table_name;

分组函数

将数据分为多个逻辑组
1.创建分组

SELECT col_name1, COUNT(*) AS num_col
FROM table_name
GROUP BY col_name1;

GROUP BY子句必须出现在WHERE子句之后，ORDER BY子句之前。

2.过滤分组
过滤分组用来决定包括哪些分组，排除哪些分组，对分组进行条件限定

SELECT col_name1, COUNT(*) AS num_col
FROM table_name
GROUP BY col_name1
HAVING COUNT(*) >= 2;

3.分组间排序

SELECT col_name1, SUM(col_name2) AS sum_col
FROM table_name
GROUP BY col_name1
HAVING SUM(col_name2) >= 50
ORDER BY sum_col;

4.关键字顺序
SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY
LIMIT