01-MySQL- SQL syntax in the case when then else end usage

SQL in the case when then else end usage

This is from someone else's blog: Address forgot

A: Simple Usage:

1. The first written:

SELECT
    s.s_id,
    s.s_name,
    s.s_sex,
    CASE
WHEN s.s_sex = '1' THEN '男' WHEN s.s_sex = '2' THEN '女' ELSE '其他' END as sex, s.s_age, s.class_id FROM t_b_student s WHERE 1 = 1

2. The second wording

SELECT
    s.s_id,
    s.s_name,
    s.s_sex,
    CASE s.s_sex
WHEN '1' THEN '男' WHEN '2' THEN '女' ELSE '其他' END as sex, s.s_age, s.class_id FROM t_b_student s WHERE 1 = 1

Two operating results as:

　　Note that there is a problem, Case function returns only the first matching value, and the remaining part will be automatically ignored Case

II: group

Table data:

t_b_population

sql statement:

SELECT country,
    SUM(CASE WHEN p.sex = '1' THEN p.population ELSE 0 END) AS '男', SUM(CASE WHEN p.sex = '2' THEN p.population ELSE 0 END) AS '女' FROM t_b_population p GROUP BY country;

operation result:

Table data:

t_b_country

sql statement:

SELECT
    SUM(c.population) AS '人口', CASE c.country WHEN '中国' THEN '亚洲' WHEN '印度' THEN '亚洲' WHEN '日本' THEN '亚洲' WHEN '美国' THEN '北美洲' WHEN '加拿大' THEN '北美洲' WHEN '墨西哥' THEN '北美洲' ELSE '其他' END AS '洲' FROM t_b_country c GROUP BY CASE c.country WHEN '中国' THEN '亚洲' WHEN '印度' THEN '亚洲' WHEN '日本' THEN '亚洲' WHEN '美国' THEN '北美洲' WHEN '加拿大' THEN '北美洲' WHEN '墨西哥' THEN '北美洲' ELSE '其他' END;

运行结果：

这是来源于其他人的博客：地址忘了

一：简单用法：

1.第一种写法：

SELECT
    s.s_id,
    s.s_name,
    s.s_sex,
    CASE
WHEN s.s_sex = '1' THEN '男' WHEN s.s_sex = '2' THEN '女' ELSE '其他' END as sex, s.s_age, s.class_id FROM t_b_student s WHERE 1 = 1

2.第二种写法

SELECT
    s.s_id,
    s.s_name,
    s.s_sex,
    CASE s.s_sex
WHEN '1' THEN '男' WHEN '2' THEN '女' ELSE '其他' END as sex, s.s_age, s.class_id FROM t_b_student s WHERE 1 = 1

两个运行结果一样：

　　有一个需要注意的问题，Case函数只返回第一个符合条件的值，剩下的Case部分将会被自动忽略

二：分组

表数据：

t_b_population

sql语句：

SELECT country,
    SUM(CASE WHEN p.sex = '1' THEN p.population ELSE 0 END) AS '男', SUM(CASE WHEN p.sex = '2' THEN p.population ELSE 0 END) AS '女' FROM t_b_population p GROUP BY country;

运行结果：

表数据：

t_b_country

sql语句：

SELECT
    SUM(c.population) AS '人口', CASE c.country WHEN '中国' THEN '亚洲' WHEN '印度' THEN '亚洲' WHEN '日本' THEN '亚洲' WHEN '美国' THEN '北美洲' WHEN '加拿大' THEN '北美洲' WHEN '墨西哥' THEN '北美洲' ELSE '其他' END AS '洲' FROM t_b_country c GROUP BY CASE c.country WHEN '中国' THEN '亚洲' WHEN '印度' THEN '亚洲' WHEN '日本' THEN '亚洲' WHEN '美国' THEN '北美洲' WHEN 'Canada ' THEN ' North America ' the WHEN ' Mexico ' THEN ' North America ' ELSE ' other ' END;

operation result:

01-MySQL- SQL syntax in the case when then else end usage

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