MySql - can I create a Calculated Field and index it?

I have a WHERE clause that looks like:
WHERE Month(PA.EntryDate) = Month('11-10-04') and Year(PA.EntryDate) = Year('11-10-04')

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Something like that shows up a lot in my DB.

I would like to add a column
EntryMonth as Month(EntryDate) -- or something to that effect.  In MS SQL Server, that would allow me to index the EntryMonth so that an index could be used in the WHERE clause.

Can I do something similar in MySql?  Every Google search I try on Calculated Fields is coming up with fields updated by a trigger ... not a true calculated field.  Is that the best MySql 5 can offer currently?

Thanks!
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Daniel WilsonAsked:
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ralmadaCommented:
1 interval month  should be interval 1 month  

WHERE PA.EntryDate >= last_day('11-10-04' - interval 1 month) + INTERVAL 1 DAY and PA.EntryDate < last_day('11-10-04') + INTERVAL 1 DAY

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ralmadaCommented:
why don't you just index EntryDate and then change your WHERE clause like this?

WHERE PA.EntryDate between dateadd(m, datediff(m, 0, '11-10-04'), 0) and dateadd(m, datediff(m, 0, '11-10-04'), 1)-1
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Daniel WilsonAuthor Commented:
select * from wt_PaymentAgreement PA WHERE PA.EntryDate between dateadd(m, datediff(m, 0, '11-10-04'), 0) and dateadd(m, datediff(m, 0, '11-10-04'), 1)-1;

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gives me this:
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' '11-10-04'), 0) and dateadd(m, datediff(m, 0, '11-10-04'), 1)-1' at line 1

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And ... what is the dateadd (datediff () ) stuff doing?  

This may be a good solution, but I do need to understand it!

Thanks!
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ralmadaCommented:
for mysql

use

WHERE PA.EntryDate between last_day('11-10-04' - 1 interval month) + 1 and last_day('11-10-04')
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Daniel WilsonAuthor Commented:
The DateDiff call is throwing it:
select datediff(m, 0, '2011-10-04');

gives the same error.

I think the syntax you gave me (once we fix whatever the error is) is saying "any date from the first of this month through 1 day less than the first of next month".

right?
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Daniel WilsonAuthor Commented:
With the last_day syntax:

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'interval month) + 1 and last_day('11-10-04')' at line 1
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ralmadaCommented:
problem is the the original suggestion was for MS SQL Server. for MySQL try http:#a36917988

>>I think the syntax you gave me (once we fix whatever the error is) is saying "any date from the first of this month through 1 day less than the first of next month".<<

correct :) basically is a way to obtain the first and last day of the month and then use those to compare. Now if you have time portion involved as well then you might want to just compare to the first day of the next month

WHERE PA.EntryDate >= last_day('11-10-04' - 1 interval month) + 1 and PA.EntryDate < last_day('11-10-04') + 1

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ralmadaCommented:
>>ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'interval month) + 1 and last_day('11-10-04')' at line 1
<<

oops, I've inverted things there:


WHERE PA.EntryDate >= last_day('11-10-04' - interval 1 month) + 1 and PA.EntryDate < last_day('11-10-04') + 1

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Daniel WilsonAuthor Commented:
I do have the time portion as well ... so ... the +1 makes sense, except it's not working right either.
mysql> select last_day('11-10-04');
+----------------------+
| last_day('11-10-04') |
+----------------------+
| 2011-10-31           |
+----------------------+
1 row in set (0.00 sec)

mysql> select last_day('11-10-04')+1;
+------------------------+
| last_day('11-10-04')+1 |
+------------------------+
|               20111032 |
+------------------------+
1 row in set (0.00 sec)

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But the biggest problem in here is the - 1 interval month  That is not being accepted at all.

should I use date_sub() ?
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Daniel WilsonAuthor Commented:
Thanks!
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