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MariaDB [demo]> select 'a' or 'b';
+------------+
| 'a' or 'b' |
+------------+
| 0 |
+------------+
1 row in set, 4 warnings (0.00 sec)
MariaDB [demo]> select 'a' and 'b';
+-------------+
| 'a' and 'b' |
+-------------+
| 0 |
+-------------+
1 row in set, 3 warnings (0.00 sec)
MariaDB [demo]> select 'a' or 'b';
+------------+
| 'a' or 'b' |
+------------+
| 0 |
+------------+
1 row in set, 4 warnings (0.00 sec)
MariaDB [demo]> show warnings;
+---------+------+----------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------+
| Warning | 1292 | Truncated incorrect INTEGER value: 'a' |
| Warning | 1292 | Truncated incorrect INTEGER value: 'b' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'b' |
+---------+------+----------------------------------------+
4 rows in set (0.00 sec)
MariaDB [demo]> set sql_mode = 'TRADITIONAL';
Query OK, 0 rows affected (0.00 sec)
MariaDB [demo]> select 'a' or 'b';
+------------+
| 'a' or 'b' |
+------------+
| 0 |
+------------+
1 row in set, 4 warnings (0.00 sec)
MariaDB [demo]> show warnings;
+---------+------+----------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------+
| Warning | 1292 | Truncated incorrect INTEGER value: 'a' |
| Warning | 1292 | Truncated incorrect INTEGER value: 'b' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'b' |
+---------+------+----------------------------------------+
4 rows in set (0.00 sec)
MariaDB [demo]> set sql_mode = 'ANSI';
Query OK, 0 rows affected (0.00 sec)
MariaDB [demo]> select 'a' or 'b';
+------------+
| 'a' or 'b' |
+------------+
| 0 |
+------------+
1 row in set, 4 warnings (0.00 sec)
MariaDB [demo]> show warnings;
+---------+------+----------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------+
| Warning | 1292 | Truncated incorrect INTEGER value: 'a' |
| Warning | 1292 | Truncated incorrect INTEGER value: 'b' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'a' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'b' |
+---------+------+----------------------------------------+
4 rows in set (0.00 sec)
MariaDB [demo]> select * from t;
+----+------+------+
| id | val | val2 |
+----+------+------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | NULL |
| 6 | 1 | 1 |
| 7 | 2 | 2 |
| 8 | 3 | 3 |
| 9 | 4 | 4 |
| 10 | 5 | 5 |
+----+------+------+
10 rows in set (0.00 sec)
MariaDB [demo]> select * from t group by id;
+----+------+------+
| id | val | val2 |
+----+------+------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | NULL |
| 6 | 1 | 1 |
| 7 | 2 | 2 |
| 8 | 3 | 3 |
| 9 | 4 | 4 |
| 10 | 5 | 5 |
+----+------+------+
10 rows in set (0.00 sec)
MariaDB [demo]> select * from t group by val;
+----+------+------+
| id | val | val2 |
+----+------+------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | NULL |
+----+------+------+
5 rows in set (0.00 sec)
MariaDB [demo]> select * from t group by val2;
+----+------+------+
| id | val | val2 |
+----+------+------+
| 1 | 1 | NULL |
| 6 | 1 | 1 |
| 7 | 2 | 2 |
| 8 | 3 | 3 |
| 9 | 4 | 4 |
| 10 | 5 | 5 |
+----+------+------+
6 rows in set (0.00 sec)
[/quote]
一般用来求某个数据中,最新的数据,比如张三 ,在1号上传了1条数据,3号上传了一条数据,
通过这个语法,可以比较容易的求出张三在3号上传的数据
MariaDB [demo]> select * from t;
+----+------+------+
| id | val | val2 |
+----+------+------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | NULL |
| 6 | 1 | 1 |
| 7 | 2 | 2 |
| 8 | 3 | 3 |
| 9 | 4 | 4 |
| 10 | 5 | 5 |
+----+------+------+
10 rows in set (0.00 sec)
MariaDB [demo]> select * from t group by id;
+----+------+------+
| id | val | val2 |
+----+------+------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | NULL |
| 6 | 1 | 1 |
| 7 | 2 | 2 |
| 8 | 3 | 3 |
| 9 | 4 | 4 |
| 10 | 5 | 5 |
+----+------+------+
10 rows in set (0.00 sec)
MariaDB [demo]> select * from t group by val;
+----+------+------+
| id | val | val2 |
+----+------+------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | 3 | NULL |
| 4 | 4 | NULL |
| 5 | 5 | NULL |
+----+------+------+
5 rows in set (0.00 sec)
MariaDB [demo]> select * from t group by val2;
+----+------+------+
| id | val | val2 |
+----+------+------+
| 1 | 1 | NULL |
| 6 | 1 | 1 |
| 7 | 2 | 2 |
| 8 | 3 | 3 |
| 9 | 4 | 4 |
| 10 | 5 | 5 |
+----+------+------+
6 rows in set (0.00 sec)