Shantanu's Blog

Every table in your database should have a primary key. A table without the primary key is like an orphan child without the support of parents or a youth without a spouse. You never know what you are missing!

1) You can easily check if the table has a primary key or not.
show create table tblName;

2) Once you have found that there is no primary key, you must check if a column or a group of columns can be declared as a primary key.
ALTER TABLE `tblName` ADD PRIMARY KEY ( `myID` , `yourID` , `type` )

3) If there is no such candidate column or a group of columns that is Unique and not null, you can simply add an auto-incremented column as shown below.
ALTER TABLE `tblName` add column autoID int not null auto_increment, add primary key (autoID)

4) If you want to remove the newly added column autoID, you can simply use the following statement.
ALTER TABLE `tblName` drop primary key, drop column autoID

This simple task can dramatically improve the performance and will make it compliant without affecting the application in any way.

Use the following shell script to find out the candidate column that can be declared as Primary.

# cat primary.sh

#!/bin/sh
# the script to find out the tables those do not have a primary key
# It will show the max and min values stored in each column along with the create table output
# change the dbname to the database name that you want to analyse
dbname='shantanu'
> /home/develop/tosave.txt

while read mytable
do
mysql shantanu -Bse"show create table $mytable\G" >> /home/develop/tosave.txt
mysql shantanu -Bse"select * from $mytable procedure analyse()\G" >> /home/develop/tosave.txt

done << heredoc
`mysql -Bse"select t.table_name as tbl from INFORMATION_SCHEMA.TABLES AS t LEFT JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS c ON (t.TABLE_NAME=c.TABLE_NAME AND c.CONSTRAINT_SCHEMA=t.TABLE_SCHEMA AND constraint_name='PRIMARY') WHERE t.table_schema='$dbname' AND constraint_name IS NULL;"`
heredoc

_____

== need for Primary Key ==

It is interesting to know what happens if we do not have a primary key to uniquely differnetiate the rows.
Here is an example table.

mysql> select * from hari;
+------+------------+-------+
| id | mydate | email |
+------+------------+-------+
| 1 | 2009-10-10 | 1111 |
| 1 | 2009-10-10 | 1111 |
| 1 | 2009-10-10 | 2222 |
| 2 | 2010-11-11 | 3333 |
| 2 | 2010-11-11 | 3333 |
+------+------------+-------+
5 rows in set (0.00 sec)

The problem with the above table is that the first 2 and last 2 rows are actually duplicate and there is no way to identify them since there is no unique or primary key used in the table.
Here is the SQL code that you can run to create this test table.

drop table if exists hari;
CREATE TABLE `hari` (
`id` int(11) default NULL,
`mydate` date default NULL,
`email` varchar(100) default NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
INSERT INTO `hari` VALUES (1,'2009-10-10','1111'),(1,'2009-10-10','1111'),(1,'2009-10-10','2222'),(2,'2010-11-11','3333'),(2,'2010-11-11','3333');

In order to get the count of the unique ID's we need the following query.

SELECT
count(sel2.refid2 ) as recCount,
refid2, recDate2
FROM
(SELECT id AS refid2, mydate AS recDate2
FROM hari AS ol_Email1
GROUP BY ol_Email1.id, ol_Email1.mydate, ol_Email1.email
) AS sel2
GROUP BY sel2.refid2;

+----------+--------+------------+
| recCount | refid2 | recDate2 |
+----------+--------+------------+
| 2 | 1 | 2009-10-10 |
| 1 | 2 | 2010-11-11 |
+----------+--------+------------+
2 rows in set (0.00 sec)

The query shown above is ugly, confusing and slow since the indexes will not be used properly. So I add a primary key to the table.

ALTER TABLE hari ADD COLUMN pid INT NOT NULL AUTO_INCREMENT, ADD PRIMARY KEY (pid);

I get the same results as shown above, but with much cleaner query that is also using the primary key.

select count(*) as ercCount, a.id as refid2, a.mydate as recDate2 from hari as a inner join
(SELECT pid, id, mydate FROM hari GROUP BY id, mydate, email) as dt
on a.pid = dt.pid group by a.id;

+----------+--------+------------+
| ercCount | refid2 | recDate2 |
+----------+--------+------------+
| 2 | 1 | 2009-10-10 |
| 1 | 2 | 2010-11-11 |
+----------+--------+------------+
2 rows in set (0.00 sec)

Labels: mysql tips, shell script