MySQL區間分組查詢

假設a表爲會員信息表，須要統計男性會員年齡各階段的出現的人數

CREATE TABLE `a` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL DEFAULT '' COMMENT '會員名稱',
  `sex` tinyint(1) unsigned NOT NULL DEFAULT '0' COMMENT '性別，一、男 二、女',
  `age` tinyint(3) unsigned NOT NULL DEFAULT '0' COMMENT '年齡',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

假設如今數據庫中有數據以下：

方法一：

SELECT
    ELT(
        INTERVAL (age, 0, 20, 30, 40),
        "1-20",
        "21-30",
        "31-40",
        "40+"
    ) AS age_area,
    COUNT(name) AS num
FROM
    `a`
WHERE
    sex = 1
GROUP BY
    ELT(
        INTERVAL (age, 0, 20, 30, 40),
        "1-20",
        "21-30",
        "31-40",
        "40+"
    );

說明：

• 利用 interval 劃出4個區間
• 再利用 elt 函數將4個區間分別返回一個列名

方法二：

SELECT
    (
        CASE
        WHEN age >= 1
        AND age <= 20 THEN
            "1-20"
        WHEN age > 20
        AND age <= 30 THEN
            "21-30"
        WHEN age > 30
        AND age <= 40 THEN
            "31~40"
        ELSE
            "40+"
        END
    ) AS age_area,
    count(name) AS num
FROM
    a
WHERE
    sex = 1
GROUP BY
    (
        CASE
        WHEN age >= 1
        AND age <= 20 THEN
            "1-20"
        WHEN age > 20
        AND age <= 30 THEN
            "21-30"
        WHEN age > 30
        AND age <= 40 THEN
            "31~40"
        ELSE
            "40+"
        END
    );

結果：