MySQL 查询数据
SELECT 的基础：选列、起别名、去重、限制条数。
Key PointsSELECT 是 SQL 里用得最多的语句；先 SELECT 列再决定过滤与排序。
选列与别名
SQL
SELECT id, username, age
FROM users;
SELECT username AS name, age AS 年龄
FROM users;Output
+----+----------+-----+
| id | username | age |
+----+----------+-----+
| 1 | alice | 28 |
| 2 | bob | 32 |
+----+----------+-----+去重与限制
SQL
SELECT DISTINCT age FROM users; -- 去重
SELECT * FROM users LIMIT 10; -- 前 10 条
SELECT * FROM users LIMIT 5 OFFSET 10; -- 跳过 10 取 5Output
+-----+
| age |
+-----+
| 28 |
| 32 |
| 25 |
| 41 |
+-----+算列与函数
SQL
SELECT username, CONCAT(username, '@', age) AS tag
FROM users;Output
+----------+-----------------+
| username | tag |
+----------+-----------------+
| alice | alice@28 |
+----------+-----------------+Avoid
SELECT * on large tables — it reads every column. Select only what you need, and paginate with LIMIT offset, size.选列与别名
SQL
SELECT id, username, age
FROM users;
SELECT username AS name, age AS 年龄
FROM users;Output
+----+----------+-----+
| id | username | age |
+----+----------+-----+
| 1 | alice | 28 |
| 2 | bob | 32 |
+----+----------+-----+去重与限制
SQL
SELECT DISTINCT age FROM users; -- 去重
SELECT * FROM users LIMIT 10; -- 前 10 条
SELECT * FROM users LIMIT 5 OFFSET 10; -- 跳过 10 取 5Output
+-----+
| age |
+-----+
| 28 |
| 32 |
| 25 |
| 41 |
+-----+算列与函数
SQL
SELECT username, CONCAT(username, '@', age) AS tag
FROM users;Output
+----------+-----------------+
| username | tag |
+----------+-----------------+
| alice | alice@28 |
+----------+-----------------+线上查询别写
SELECT * 后 LIMIT 全表——SELECT * 会读出所有列（含大 TEXT），只取需要的列更省 IO。分页用 LIMIT offset, size。