This post looks slightly different to earlier ones as I created it with the verbose option and ran the SQL from a file using the source command. MySQL allows you to increment column values automatically with the auto_increment clause. You can only auto increment one column per table and that column must be used as a key:
--------------
create table andrews_table(
id int(4) auto_increment,
location varchar(10))
--------------
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
--------------
create table andrews_table(
id int(4) auto_increment,
location varchar(10),
primary key(id))
--------------
Query OK, 0 rows affected (0.00 sec)
When
you insert rows into the table, you do not need to supply values for
the column which is automatically incremented. Note how three rows are
being inserted simultaneously:
--------------
insert into andrews_table (location)
values ('Redhill'), ('Crawley'), ('Horsham')
--------------
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
By
default, it looks as if the auto incremented column starts at 1 and
goes up by 1 each time. I will look at this in more detail in future
posts:
--------------
select * from andrews_table
--------------
+----+----------+
| id | location |
+----+----------+
| 1 | Redhill |
| 2 | Crawley |
| 3 | Horsham |
+----+----------+
3 rows in set (0.01 sec)
MySQL
allows you to look at the value automatically inserted into the last
row. For a multi-row insert, this returns the value inserted into the
first row created by that insert:
--------------
select last_insert_id()
--------------
+------------------+
| last_insert_id() |
+------------------+
| 1 |
+------------------+
1 row in set (0.00 sec)
--------------
insert into andrews_table (location)
values ('Croydon')
--------------
Query OK, 1 row affected (0.00 sec)
--------------
select * from andrews_table
--------------
+----+----------+
| id | location |
+----+----------+
| 1 | Redhill |
| 2 | Crawley |
| 3 | Horsham |
| 4 | Croydon |
+----+----------+
4 rows in set (0.00 sec)
--------------
select last_insert_id()
--------------
+------------------+
| last_insert_id() |
+------------------+
| 4 |
+------------------+
1 row in set (0.00 sec)
mysql>
