數據庫Ⅴ——查詢數據操作（2）

一、ORDER BY對查詢結果排序

1.升序

• 按照id降序排列DESC ,默認的是升序ASC

SELECT * FROM cms_user ORDER BY id ;
SELECT * FROM cms_user ORDER BY id ASC;
SELECT * FROM cms_user ORDER BY id DESC;

• 按照年齡升序排列

SELECT * FROM cms_user ORDER BY age ASC;

2.降序

• 按字段位置降序排列

SELECT * FROM cms_user ORDER BY 1 DESC;

更新數據

UPDATE cms_user SET age=12 WHERE id=5;

3.升序、降序同時使用

• 按照年齡升序，id降序排列

SELECT * FROM cms_user ORDER BY age ASC,id DESC;
#查詢id,age,sex，顯示分組詳情，統計各組人數和各組年齡總和，並且限制條件id>=2 ，按照sex分組，每組人數大於等於2，並且按照年齡降序，id升序排列
SELECT id,age,sex,GROUP_CONCAT(username),COUNT(*) AS totalUsers,SUM(age) AS sum_age 
FROM cms_user 
WHERE id>=2 
GROUP BY sex
HAVING COUNT(*)>=2
ORDER BY age DESC,id ASC;

– 實現記錄隨機

SELECT * FROM cms_user ORDER BY RAND();

二、LIMIT限制顯示條數

1.查詢前n條記錄

• 查詢表中前3條記錄

SELECT * FROM cms_user LIMIT 3;

• 查詢按id降序排列的前五條記錄

SELECT * FROM cms_user ORDER BY id DESC LIMIT 5;

2.根據（起始位置，步長）查詢記錄

• 查詢表中前一條記錄（起始位置，步長）

SELECT * FROM cms_user LIMIT 1;
SELECT * FROM cms_user LIMIT 0,1;
SELECT * FROM cms_user LIMIT 1,1;
SELECT * FROM cms_user LIMIT 0,5;

• 查詢id,age,sex，顯示分組詳情，統計各組人數和各組年齡最大值、最小值、平均值、總和，並且限制條件id>=1 ，按照sex分組，每組人數大於等於2，並且按照年齡降序排列，取前兩條記錄

SELECT id,sex,age,GROUP_CONCAT(username),
COUNT(*) AS totalUsers,
MAX(age) AS max_age,
MIN(age) AS min_age,
AVG(age) AS avg_age,
SUM(age) AS sum_age
FROM cms_user
WHERE id>=1
GROUP BY sex
HAVING COUNT(*)>=2
ORDER BY age DESC
LIMIT 0,2;