using if statement in mysql query - mysql

Question

I would like to use if statement in sql query :
what I want :
if(tractions_delivery.send_date_id !=0 ){
date_send_commodities.id = tractions_delivery.send_date_id
}
my query :
from
tractions_delivery,user_address,province,city,date_send_commodities,users
WHERE
tractions_delivery.tr_id = $tr_id
AND
tractions_delivery.address_id = user_address.id
AND
user_address.province_id = province.id
AND
user_address.city_id = city.id
AND
//not work
(tractions_delivery.send_date_id IS NOT 0 date_send_commodities.id = tractions_delivery.send_date_id)
AND
users.id = user_address.user_id

Answer 1

You could use the CASE-statement
SELECT
*
FROM
tractions_delivery,
user_address,
province,
city,
date_send_commodities,users
WHERE
tractions_delivery.tr_id = $tr_id AND
tractions_delivery.address_id = user_address.id AND
user_address.province_id = province.id AND
user_address.city_id = city.id AND
CASE WHEN tractions_delivery.send_date_id != 0 THEN date_send_commodities.id = tractions_delivery.send_date_id ELSE 1=1 END AND
users.id = user_address.user_id

Answer 2

You can only use if statements in stored procedures or functions. If you just write a sql statement unfortunately you cannot use if statements around the query. But you can use logic in the query itself, e.g.:
SELECT CASE WHEN col1 = col2 THEN'col1 equals col2' else 'col1 doesnt equal col2' ELSE
FROM table1
So around doesnt work, but in the field list you can create CASE WHEN ELSE END logic.

Answer 3

CASE or IF() operators can be of help.
Examples,
SELECT (CASE 1 WHEN 1 THEN 'One' WHEN 2 THEN 'Two' ELSE 'More' END) 'Result';
OR
SELECT IF(1=1, 'One', 'Two') 'Result';
These CASE and IF() operators can be used in the SELECT clause to conditionally interpret column values and return in the resultset.
Note: Do not confuse CASE operator here with 'CASE conditional syntax block' that ends with END CASE.