〇、概述

1、內容

選擇（雙分支、多分支）

一、條件函式

1、計算25歲以上和以下的使用者數量

CASE THEN END多分支選擇

SELECT
    (CASE 
        WHEN age>=25 THEN '25歲及以上'
        ELSE '25歲以下'
    END) AS age_cut,
    COUNT(*) AS number    
FROM user_profile
GROUP BY age_cut;

IF ELSE雙分支選擇

SELECT
    IF(age>=25,'25歲及以上','25歲以下') AS age_cut,
    
 
COUNT(*) AS number
FROM user_profile
GROUP BY age_cut;

集合運算聯合UNION

SELECT
    '25歲以下' AS age_cut,
    COUNT(*) AS number
FROM user_profile
WHERE 
    age<25
    or
    age IS NULL
UNION
SELECT
    '25歲及以上' AS age_cut,
    COUNT(*) AS number
FROM user_profile
WHERE age>=25;

2、檢視不同年齡段的使用者明細

CASE WHEN THEN END，中間不加逗號

SELECT
    device_id,
    gender,
    (CASE 
         WHEN age IS NULL THEN '其他'
         WHEN age<20 THEN '20歲以下'
         WHEN age BETWEEN 20 and 24 THEN '20-24歲'
         ELSE '25歲及以上'
    END) AS age_cut
FROM user_profile;

二、日期函式

三、文字函式