MySQL查询语句是用于从数据库中选择数据的基本工具，其基本语法结构如下：
```sql
SELECT column1, column2, ...
FROM table_name
[WHERE condition]
[GROUP BY column1, column2, ...]
[HAVING condition]
[ORDER BY column1 ASC|DESC]
[LIMIT M, N]
```
基础查询
查询所有字段：`SELECT * FROM table_name；`
查询指定字段：`SELECT column1, column2 FROM table_name；`
使用 AS 给字段起别名：`SELECT column1 AS alias1, column2 AS alias2 FROM table_name；`
条件查询
使用 WHERE 子句筛选数据：`SELECT * FROM table_name WHERE condition；`
示例：`SELECT name, age FROM user WHERE age > 18；`
逻辑运算符：`AND`, `OR`, `NOT`
示例：`SELECT * FROM student WHERE age >= 20 AND score > 50；`
排序和限制
使用 ORDER BY 对查询结果进行排序：`SELECT * FROM table_name ORDER BY column_name ASC|DESC；`
示例：`SELECT name, score FROM student ORDER BY score DESC LIMIT 3；`
分组查询
使用 GROUP BY 按指定列分组：`SELECT column1, AVG（column2） FROM table_name GROUP BY column1；`
示例：`SELECT class_id, AVG（score） AS avg_score FROM student GROUP BY class_id；`
聚合函数
COUNT：`SELECT COUNT（*） FROM table_name；`
SUM：`SELECT SUM（column_name） FROM table_name；`
AVG：`SELECT AVG（column_name） FROM table_name；`
MAX：`SELECT MAX（column_name） FROM table_name；`
MIN：`SELECT MIN（column_name） FROM table_name；`
多表查询
子查询：`SELECT * FROM table1 WHERE column_name IN （SELECT column_name FROM table2）；`
联表查询（JOIN）：`SELECT t1.column1, t2.column2 FROM table1 t1 INNER JOIN table2 t2 ON t1.column_name = t2.column_name；`
其他
LIKE：用于模糊匹配，`LIKE 'pattern%'`
REGEXP：用于正则表达式匹配，`REGEXP 'pattern'`
示例
查询所有字段
```sql
SELECT * FROM user；
```
查询指定字段
```sql
SELECT name, age FROM user；
```
使用 AS 起别名
```sql
SELECT name AS user_name, age AS user_age FROM user；
```
条件查询
```sql
SELECT * FROM user WHERE age > 18；
```
排序和限制
```sql
SELECT name, score FROM student ORDER BY score DESC LIMIT 3；
```
分组查询
```sql
SELECT class_id, AVG（score） AS avg_score FROM student GROUP BY class_id；
```
聚合函数
```sql
SELECT COUNT（*） FROM student；
SELECT SUM（score） AS total_score FROM student；
SELECT AVG（score） AS average_score FROM student；
SELECT MAX（score） AS highest_score FROM student；
SELECT MIN（score） AS lowest_score FROM student；
```
多表查询 - 子查询
```sql
SELECT * FROM orders WHERE order_id IN （SELECT order_id FROM customers WHERE city = 'New York'）；
```
多表查询 - 联表查询
```sql
SELECT customers.name, orders.order_date
FROM customers
INNER JOIN orders ON customers.customer_id = orders.customer_id；
```
这些查询语句可以帮助你高效地从MySQL数据库中检索所需的数据。根据实际需求选择合适的查询语句和条件，可以大大提高查询效率和数据处理的准确性。
