MySql学习的一大重点：基本查询

-- 仅向指定列插入数据，未指定的列取默认值或NULL
INSERT INTO students (id, sn, name) 
VALUES (10, 124, '关羽');

-- 必须严格匹配表定义的列顺序，传入所有列的值
INSERT INTO students 
VALUES (1, 123, '张飞', '4567890');

-- 单次提交多行数据，性能远高于多次单行插入
INSERT INTO students (id, sn, name, qq)
VALUES 
  (13, 127, '许攸', '10423456'),
  (14, 128, '许褚', '12333445'),
  (15, 129, '诸葛亮', '3213456');

-- id=18为主键，若已存在则更新sn、name、qq字段
INSERT INTO students (id, sn, name, qq)
VALUES (18, 134, '貂蝉', '1112222')
ON DUPLICATE KEY UPDATE 
  sn = 134,
  name = '貂蝉',
  qq = '11133333';
-- 查看上一次DML语句影响的行数
SELECT row_count();

-- sn为唯一键，若140已存在则删除旧行后插入新行
REPLACE INTO students (sn, name, qq)
VALUES (140, '许攸', '321456213');

-- 全列查询（不推荐）
SELECT * FROM exam_result;
-- 指定列查询
SELECT id, name, math FROM exam_result;

-- 计算三科总分并设置别名total
SELECT 
  name,
  math,
  chinese + math + english AS total 
FROM exam_result;
-- 省略AS的别名写法
SELECT 
  name 姓名,
  math 数学,
  chinese + math + english 总分 
FROM exam_result;

-- 对数学成绩去重，相同分数仅保留一条
SELECT DISTINCT math FROM exam_result;

-- 报错：Unknown column 'total' in 'where clause'
SELECT name, chinese + math + english total 
FROM exam_result 
WHERE total < 200;

-- WHERE中直接书写完整表达式
SELECT name, chinese + math + english 总分 
FROM exam_result 
WHERE chinese + math + english < 200;

-- 英语成绩低于60分的同学
SELECT name, english FROM exam_result WHERE english < 60;
-- 语文成绩在[80,90]分（BETWEEN闭区间写法）
SELECT name, chinese FROM exam_result WHERE chinese BETWEEN 80 AND 90;
-- 数学成绩是58/59/98/99（IN集合写法）
SELECT name, math FROM exam_result WHERE math IN (58, 59, 98, 99);
-- 姓孙的同学（%模糊匹配）
SELECT name FROM exam_result WHERE name LIKE '孙%';
-- 名字为两个字且姓孙（_单字符匹配）
SELECT name FROM exam_result WHERE name LIKE '孙_';
-- 语文成绩 > 80 且不姓孙
SELECT name, chinese FROM exam_result 
WHERE chinese > 80 AND name NOT LIKE '孙%';
-- 查询name为NULL的记录
SELECT * FROM test WHERE name IS NULL;

-- 数学成绩升序（默认ASC）
SELECT name, math FROM exam_result ORDER BY math ASC;
-- 数学成绩降序
SELECT name, math FROM exam_result ORDER BY math DESC;

-- 先按数学降序，数学同分则按英语升序，再同分按语文升序
SELECT name, math, english, chinese 
FROM exam_result 
ORDER BY math DESC, english ASC, chinese ASC;

-- 使用总分别名进行降序排序
SELECT name, chinese + math + english total 
FROM exam_result 
ORDER BY total DESC;

-- 1. 取前3条结果
SELECT * FROM exam_result LIMIT 3;
-- 2. 从下标1开始，取3条（对应第2~4行数据）
SELECT * FROM exam_result LIMIT 1, 3;
-- 3. 标准写法：取3条，偏移量为1（语义更清晰，推荐使用）
SELECT * FROM exam_result LIMIT 3 OFFSET 1;

-- 每页3条，查询第2页数据
SELECT id, name, math, english, chinese 
FROM exam_result
ORDER BY id 
LIMIT 3 OFFSET 3;