Mysql - Concat multiple INT fields together to make DATE field - mysql

Question

I am working ona small project for a client, and the data in the database is causing me a bit of bother. I am completing a search form for paid installments from the database. I can get the data fine, its the date fields which are giving me bother.
There are separate fields for the date, month and year for each installments. Is there a way I can use CONCAT or DATE or maybe even use CAST on the combined fields to search for the data.
I have tried the following
SELECT DATE_FORMAT( CONCAT( received_date, received_month, received_year ) , '%d/%m/%Y' ) FROM policy_installments
AND
SELECT DATE_FORMAT(STR_TO_DATE(CONCAT('pit.received_date', '-', 'pit.received_month','-', 'pit.received_year'), '%d-%m-%Y') AS mydate, '%d-%m-%Y') AS modate FROM policy_installments pit ORDER BY mydate
AND
SELECT * FROM policy_installments WHERE CONCAT(CAST(received_date AS CHAR),"-",CAST(received_month AS CHAR),"-",CAST(received_year AS CHAR)) >= '1-1-2014' AND CONCAT(CAST(received_date AS CHAR),"-",CAST(received_month AS CHAR),"-",CAST(received_year AS CHAR)) <= '31-3-2014'
When I use the following I get some unexpected data:
SELECT CONCAT_WS( received_date, "-", received_month, "-", received_year ) FROM policy_installments
RESULT
2d313130312d3132303039
2d3237313032372d323732303039 etc etc
Each of the 3 fields are type INT(11).
However, none of these give me the correct data. Is there a way i can convert all 3 fields and use a date to pass into the query to enable me to correctly grab the correct data?
Any help would be much appreciated!

Answer 1

To convert a concatenated string to date, string should be in the same format as what you are using to convert.
For a date-string in dd/mm/yyyy format, should be converted to sql date using %d/%m%Y format.
Example:
SELECT STR_TO_DATE(
CONCAT( received_date, '/', received_month, '/', received_year ),
'%d/%m/%Y'
) as rcvd_date
FROM policy_installments
And to compare with other dates, you can use BETWEEN clause.
Example:
SELECT * FROM policy_installments
WHERE
STR_TO_DATE(
CONCAT( received_date, '-', received_month, '-', received_year ),
'%d-%m-%Y'
) BETWEEN STR_TO_DATE( '1-1-2014', '%d-%m-%Y' )
AND STR_TO_DATE( '31-3-2014', '%d-%m-%Y' )
Refer to:
MySQL: STR_TO_DATE(str,format): Convert a string to a date

Answer 2

Generally, you want to avoid performing the function on the column. Instead, perform the function on the literal, so that you can utilize indexes on the columns:
SELECT *
FROM policy_installments
WHERE received_year = YEAR('2014-03-31')
AND received_month = MONTH('2014-03-31')
AND received_date = DAY('2014-03-31')
Add a multi-column index on received_year, received_month, and received_date, you'll be doing pretty good.
Obviously, this method will only work for = comparisons. For <=, it becomes a bit more complicated:
SELECT *
FROM policy_installments
WHERE
received_year < YEAR('2014-03-31')
UNION
SELECT *
FROM policy_installments
WHERE
received_year = YEAR('2014-03-31')
AND received_month < MONTH('2014-03-31')
UNION
SELECT *
FROM policy_installments
WHERE
received_year = YEAR('2014-03-31')
AND received_month = MONTH('2014-03-31')
AND received_date <= DAY('2014-03-31')
But this longer query is still able to utilize the index.
To simplify things, consider creating an actual date column and keep it updated with the proper value.