Summary of delete statements in mysql (Part 2)

4. The update set statement is the most commonly used modification and update statement in MySQL. When it updates information, it will also overwrite (delete) the old information.

A update set is used with where to change certain records:

update + table name + set + changed information + where clause;

For example:

update stu set birth=1988,department='中文系' where id=9    and name='张三';

Note: If there is no where clause after the change information to specify the content of the change, then the update set statement will update and modify all the information in this field.

For example

mysql> select * from c1score;
+-------+------+
| score | s    |
+-------+------+
|    56 |    1 |
|    79 |    2 |
|    91 |    3 |
|    46 |    5 |
|    35 |    6 |
+-------+------+
5 rows in set (0.08 sec)
mysql> update c1score set score=score+8;
Query OK, 5 rows affected (0.13 sec)
Rows matched: 5  Changed: 5  Warnings: 0
mysql> select * from c1score;
+-------+------+
| score | s    |
+-------+------+
|    64 |    1 |
|    87 |    2 |
|    99 |    3 |
|    54 |    5 |
|    43 |    6 |
+-------+------+                                                                                                    
5 rows in set (0.00 sec)

5Use alter statement to delete fields:

A alter table table name drop field name;

mysql> alter table c1score drop s;
Query OK, 0 rows affected (1.80 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> select * from c1score;
+-------+
| score |
+-------+
|    64 |
|    87 |
|    99 |
|    54 |
|    43 |
+-------+
5 rows in set (0.00 sec)

B Use alter to delete the index:

mysql> drop index idx_4a on 4a;    
Query OK, 0 rows affected (0.41 sec)    
Records: 0  Duplicates: 0  Warnings: 0

C Use alter to delete the primary key:

mysql> alter table sc3 drop primary key;
Query OK, 17 rows affected (1.00 sec)
Records: 17  Duplicates: 0  Warnings: 0

D Use alter to delete and update the table name:

mysql> alter table sc3 rename to gyssc;
Query OK, 0 rows affected (0.30 sec)
mysql> select * from sc3;
ERROR 1146 (42S02): Table 'trains.sc3' doesn't exist

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