MYSQL - how to a second unique string key? - mysql

Question

Here is my goods table.
+----------------------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+---------------+------+-----+---------+-------+
| ID | decimal(18,0) | NO | PRI | | |
| gkey | varchar(255) | YES | MUL | NULL | |
| GOODS | decimal(18,0) | YES | MUL | NULL | |
Column ID is auto-increment.
GOODS is the id of a goods category.
Here is the goods category table.
+-------+---------------------+
| ID | NAME |
+-------+---------------------+
| 1 | book |
| 2 | phone |
+-------+---------------------+
My question is in goods table I need the gkey is also an unique key with prefix-id(here id is started from 1.) Like BOOK-1,BOOK-2....PHONE-1,PHONE-2... when insert a new goods record into goods table.
Like that:
+--------------------+---------------+------+-----+---------+-------+
| ID | GKEY |GOODS | PRI | COUNTRY | Extra |
+--------------------+---------------+------+-----+---------+-------+
| 1 | BOOK-1 | 1 | 10 | | |
| 2 | PHONE-1 | 2 | 12 | | |
| 3 | BOOK-2 | 1 | 13 | | |
| 4 | BOOK-3 | 1 | 10 | | |
| 5 | PHONE-2 | 2 | 10 | | |
| 6 | PHONE-3 | 2 | 20 | | |
+--------------------+---------------+------+-----+---------+-------+
How to get that GKEY in PHP+MYSQL?
thanks.

Answer 1

You can use the UNIQUE constraint. For more information check here.

Answer 2

I don't think you want to be doing this at all. Instead, good.gkey should be a foreign key to goods_category.id. If the insertion order is important, you can add an insert_date date field to the goods table.
From there you can run all sorts of queries, with the added bonus of having referential integrity on your tables.

Answer 3

First you should do everything Database side. So no php. That would be my advice.
Then not having sequence in MySQL that what I would Suggest you :
SELECT COUNT(id)
FROM GOODS
WHERE GKEY LIKE('BOOK-%')
Then you insert
INSERT INTO goods (id, "BOOK-" || SELECT MAX(SUBSTR(LENGTH(GKEY -1), LENGTH(GKEY))FROM GOODS WHERE GKEY LIKE('BOOK-%') + 1, ...)
This way you will always have the next number available.

Answer 4

You can do it, but you need to be quite careful to make sure that two simultaneous inserts don't get given the same gkey. I'd design it with these two design points:
In the GoodsCategory table, have a counter which helps to generate the next gkey.
Use locks so that only one process can generate a new key at any one time.
Here's the details:
1. Add a couple of columns to the GoodsCategory table:
+----------------------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+---------------+------+-----+---------+-------+
| ID | TINYINT | NO | PRI | | |
| NAME | VARCHAR(80) | NO | | | |
| KeyCode | CHAR(5) | NO | | | |
| NextID | INT | NO | | 0 | |
+----------------------+---------------+------+-----+---------+-------+
The KeyCode has 'BOOK' or 'PHONE', and the NextID field stores an int which is used to generate the next key of that type i.e. if the table looks like this:
+----------------+------------+---------+--------+
| ID | NAME | KeyCode | NextID |
+----------------+------------+---------+--------+
| 1 | book | BOOK | 3 |
| 2 | phone | PHONE | 7 |
+----------------+------------+---------+--------+
Then the next time you add a book, it should be given gkey 'BOOK-3', and NextID is incremented to 4.
2: The locking will need to be done in a stored routine, because it involves multiple statements in a transaction and uses local variables too. The core of it should look something like this:
START TRANSACTION;
SELECT KeyCode, NextID INTO v_kc, v_int FROM GoodsCategory WHERE ID = 2 FOR UPDATE;
SET v_newgkey = v_kc + '-' + CAST(v_int AS CHAR);
INSERT INTO goods (gkey, goods, ...) VALUES (v_newgkey, 2, etc);
UPDATE GoodsCategory SET NextID = NextID + 1 WHERE ID = 2;
COMMIT;
The FOR UPDATE bit is crucial; have a look at the Usage Examples in the mysql manual where it discusses how to use locks to generate an ID without interference from another process.