Posts Tagged ‘sql’

If you have a complex SQL query, you might find that performance isn’t exactly ideal. Worse still, you don’t actually know which part of the query it is that is actually taking so long.

Luckily, MySQL comes to the rescue with the ability to explain.

All you have to do is start your query with the keyword EXPLAIN and MySQL will, rather than returning you a recordset of results, will instead provide a break down of everything it has done, including how it made the table joins and what order it did everything in.

EXPLAIN SELECT a INNER JOIN b ON a.col1 = b.col2 WHERE a.col1 > 1 AND b.col2 > 2;

If your insert statements are not time critical, you can use insert delayed in your SQL to speed things up. The syntax is as follows.

INSERT DELAYED table (col1, col2) VALUES ('a', 'b');

Insert delayed can be used with MyISAM and Memory, but cannot be used with InnoDB.

The advantage of using insert delayed is that the MySQL server returns a success message straight away so the script can keep going, without it actually having to do the insert. This allows the MySQL server to carry it out when it isn’t busy, and do several at the same time.

It’s appropriate for tables such as logs tables where it doesn’t matter too much if they don’t go in straight away.

The SQL update command allows you to make specific changes to one or more rows. You could in theory do this by deleting the old one and inserting a new one but this means you have to deal with all the data and can only do one row of data. Using UPDATE solves both of these problems.

Basic syntax

As with everything there is a basic command structure to allow you to sort out what you want and run the various different parts of the commands. The syntax for using UPDATE is as follows:

UPDATE table SET field1 = 'value1', field2 = somenumber WHERE field3 = 'value3'

There are three parts to this. The first is the table to update. The second is the values which you are setting. The third is the validation of which rows in the table should have the update run.

Sample table

To make this easier to do I am going to create a sample table of data from which we can work from. The table is going to be called friends and will have several bits of data in it.

Name	email	age	favourite colour
Jim	jim@mail.com	24	blue
Alex	alex@operamail.com	23	black
Josh	happy@killer.com	30	grey
Mike	mikebob@avril.com	27	blue

Ok first lets say that Josh tells me that his email has changed from happy@killer.com to josh@wiggam.com. So we need to run an update command to change the email address from the old one to the new one.

UPDATE friends SET email = 'josh@wiggam.com' WHERE name = 'Josh'

In this code, all rows where the name is Josh, will have the current email changed to match josh@wiggam.com. This would be a problem if I had two Josh’s but I don’t so only one is updated. If I did have two Josh’s I could change the statement to say:

WHERE email = 'happy@killer.com'

Or I could also try

WHERE name = 'Josh' AND age = 30

Updating multiple values

Next let’s pretend that it’s Alex’s birthday – he turns 24 and decides that his favourite colour is now green. So we need to update two values in the table at once. Luckily this is pretty simple to do.

UPDATE friends SET age = 24, favourite colour = 'green' WHERE name = 'Alex'

First thing I must stress – you should never have a column name with a space it – don’t do it. But seen as this is just an example it’s not a problem. If you really needed a name like that then using FAVOURITE_COLOUR or a dash, etc to space them out.

In the above update statement both age and favourite colour are updated in every row where the name is Alex. Also there are no hyphens around age as it’s a number and so does not require them. You will get an error if you try to put them round a number when using a number column, as you will do for not using them in a text column.

Updating multiple rows

Finally I am going to look at how to update two records at a time. Though we have really already covered it so this is more of a confirmation to make sure you have it right. Take a look at this code:

UPDATE friends SET age = 60 WHERE favourite colour = 'blue'

I decide that everyone who likes blue is an old foogy so I am going to update everyone’s age to 60 who said their favourite colour was blue. As usual the script looks for everyone who is ok by the WHERE validation. This time it finds both Jim and Mike and changes both their ages to 60.

Conclusion

The UPDATE command is fairly simple – you just specify the table to update, the values to set and the conditions a row must meet for the update to be run on that row. One final thought though – take another look at Mike’s email address and try and guess what music I was listening to when I wrote that one :).

Yep, it was of course Feeder ;).

Whether you connect to MySQL, MS Access, SQL or more databases and whether you do it via PHP, ASP or even more you still use the same basic commands of SQL to extract information from the database.

Below is a sample database table called “members”

Username	Name	Email	Posts
Jim	Jim Harris	jim@ntlworld.com	24
Mod	Toby Hunter	t.hunter@btopenworld.com	4
Happy	Simon Gates	theman2003@aol.com	12

Seen as your database connections and how you phrase the code depends on what language you are using I won’t cover that. Chances are you will have some variable such as db = “your SQL commands here”

First of all, you may want to extract all the data. A basic command would be:

SELECT * FROM members

The SQL commands such as SELECT and FROM are always in capitals. SELECT is the first thing you put whenever you are reading or extracting data. It tells you what to take out. For instance you may only want certain columns.

SELECT name FROM members

Or

SELECT name,email FROM members

* tells the script to take all the columns from the database.

FROM tells you what table in the database to take the data from. So for instance if you had a table called “stats” you would use:

SELECT * FROM stats

Next you may want to add conditionals onto the data to only take certain rows:

SELECT * FROM members WHERE name = "Jim Harris"

This would give you the result:

Jim Jim Harris jim@ntlworld.com 24

You can also add several conditional values on:

SELECT * FROM members WHERE username = "jim" and name = "Jim Harris"

That would produce:

Jim Jim Harris jim@ntlworld.com 24

As with most mathematical type equations you don’t have to use the = sign all the time. For instance:

SELECT * FROM members WHERE posts > 10

This would produce

Jim Jim Harris jim@ntlworld.com 24
Happy Simon Gates theman2003@aol.com 12

The guy with the username “mod” would be missed out as his has not made more than 10 posts.

Finally you can also order the rows. For instance if you wanted to order then by the number of posts they made:

SELECT * FROM members ORDER BY posts DESC

This would produce:

Jim Jim Harris jim@ntlworld.com 24
Happy Simon Gates theman2003@aol.com 12
Mod Toby Hunter t.hunter@btopenworld.com 4

By contrast:

SELECT * FROM members ORDER BY posts ASC

Would produce:

Mod Toby Hunter t.hunter@btopenworld.com 4
Happy Simon Gates theman2003@aol.com 12
Jim Jim Harris jim@ntlworld.com 24

ASC sorted them lowest first or alphabetically and DESC sorts them highest first or reverse alphabetically.