Shantanu's Blog

Corporate Consultant

August 15, 2007

 

MySQL FAQ

1) Why doesn't the following query work?

SELECT col1, col2, concat_ws( ' ', firstname, lastname ) AS fullname
FROM players
WHERE fullname LIKE 'The Name'

The reason you are getting the error is because you can't refer to a column alias in a WHERE clause. Take your main part of the query:

SELECT concat_ws( ' ', firstname, lastname ) AS fullname
FROM players

and wrap another select around it:

SELECT col1, col2 FROM (
SELECT concat_ws( ' ', firstname, lastname ) AS fullname
FROM players)

now in order to use this you need to assign an alias to your inner query because it is considered a derived table. you can use whatever you like but using DT or dt is easy to remind you it is a Derived Table

SELECT col1, col2 FROM (
SELECT concat_ws( ' ', firstname, lastname ) AS fullname
FROM players
) as DT

You can now use your WHERE clause to refer to fullname.

SELECT col1, col2 FROM (
SELECT concat_ws( ' ', firstname, lastname ) AS fullname
FROM players
) as DT
WHERE fullname LIKE 'The Name'
LIMIT 0,30

_____

2) Why am I getting the wrong results?

SELECT ROUND(( NOW() - last_accessed ) /60)
FROM sessions
WHERE user_id = '1'

Sometimes your query may show unexpected results. For e.g. in the query you may get correct results if the difference is 1 or 2 minutes but if the difference is 20 minutes or so, you get the incorrect value of 30 minutes and if actual result is 45, you get 58 minutes and so on. The correct way to write this query is to convert the time to seconds and then divide by 60.

SELECT ROUND( TIME_TO_SEC( NOW() - last_accessed ) /60 ) AS minutes_ago
FROM sessions
WHERE user_id =171

And if you are using latest version of MySQL then you can use timediff function.

SELECT ROUND( TIME_TO_SEC(TIMEDIFF(NOW(), last_accessed) ) / 60 ) as minutes_ago
FROM sessions
WHERE user_id = 1

_____

3) What is a Theta Join?

Theta Joins are not joins which employ comma syntax, theta joins are joins which involve inequality comparisons in the join conditions. For e.g.

select a.foo, b.bar
from a, b where b.id < a.id

select a.foo, b.bar
from a inner join b on b.id < a.id

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