语法
以下是 SQL SELECT 语句使用 WHERE 子句从数据表中读取数据的通用语法：
SELECT field1, field2,...fieldN FROM table_name1, table_name2...
[WHERE condition1 [AND [OR]] condition2.....
查询语句中你可以使用一个或者多个表，表之间使用逗号, 分割，并使用WHERE语句来设定查询条件。
你可以在 WHERE 子句中指定任何条件。
你可以使用 AND 或者 OR 指定一个或多个条件。
WHERE 子句也可以运用于 SQL 的 DELETE 或者 UPDATE 命令。
WHERE 子句类似于程序语言中的 if 条件，根据 MySQL 表中的字段值来读取指定的数据。
mysql> select * from myroot;
+----+------+-----+--------+------------+
| id | name | age | phone | date |
+----+------+-----+--------+------------+
| 1 | kk | 12 | 232434 | 2022-12-03 |
+----+------+-----+--------+------------+
1 row in set (0.01 sec)
mysql> select id, name from myroot;
+----+------+
| id | name |
+----+------+
| 1 | kk |
+----+------+
1 row in set (0.00 sec)
mysql> select phone, date from myroot;
+--------+------------+
| phone | date |
+--------+------------+
| 232434 | 2022-12-03 |
+--------+------------+
1 row in set (0.00 sec)
mysql> insert into myroot (name,age, phone, date) values ('kkkk', 2234, 2232, '2033-2-32');
ERROR 1292 (22007): Incorrect date value: '2033-2-32' for column 'date' at row 1
mysql> insert into myroot (name,age, phone, date) values ('kkkk', 2234, 2232, '2033-2-01');
Query OK, 1 row affected (0.01 sec)
mysql> select * from myroot;
+----+------+------+--------+------------+
| id | name | age | phone | date |
+----+------+------+--------+------------+
| 1 | kk | 12 | 232434 | 2022-12-03 |
| 2 | kkkk | 2234 | 2232 | 2033-02-01 |
+----+------+------+--------+------------+
2 rows in set (0.00 sec)
mysql> select * from myroot where age = 2234;
+----+------+------+-------+------------+
| id | name | age | phone | date |
+----+------+------+-------+------------+
| 2 | kkkk | 2234 | 2232 | 2033-02-01 |
+----+------+------+-------+------------+
1 row in set (0.01 sec)
mysql> select age from myroot;
+------+
| age |
+------+
| 12 |
| 2234 |
+------+
2 rows in set (0.00 sec)
mysql> select age = 2234 from myroot;
+------------+
| age = 2234 |
+------------+
| 0 |
| 1 |
+------------+
2 rows in set (0.00 sec)
mysql> select * from myroot;
+----+------+------+--------+------------+
| id | name | age | phone | date |
+----+------+------+--------+------------+
| 1 | kk | 12 | 232434 | 2022-12-03 |
| 2 | kkkk | 2234 | 2232 | 2033-02-01 |
+----+------+------+--------+------------+
2 rows in set (0.00 sec)
mysql> select * from myroot where age > 12;
+----+------+------+-------+------------+
| id | name | age | phone | date |
+----+------+------+-------+------------+
| 2 | kkkk | 2234 | 2232 | 2033-02-01 |
+----+------+------+-------+------------+
1 row in set (0.01 sec)