Determine column data with if condition in a joined tables SQL - mysql

Question

I have a query that works well, where i would like an assistance is how to incorporate a condition to determine the value displayed on a given column. That is if a column of a table has a value say "Authorization of COA" then the column date added should show the date else the column should show null
Here is my query so far
SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' , t.date_added (//should show the value of date_added else NULL based on a condition for this column),
FROM request r
LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
WHERE r.client_id='$cid'
AND r.created_at BETWEEN '$start' AND '$end'
AND a_s.department_id = '$dept'
GROUP BY r.request_id
ORDER BY `r`.`created_at` DESC "

Answer 1

A simple CASE statement should be sufficient in this case, I believe.
SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id,
CONCAT(u.fname,' ',u.lname) 'analyst',
CASE WHEN ConditionColumn = 'Authorization of COA' THEN t.date_added
ELSE NULL END AS 'date_added'
FROM request r
LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
WHERE r.client_id='$cid'

Answer 2

In both mysql and ms sql you can use case expression (mysql case; ms sql case) to assign value to a field conditionally:
SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' ,
CASE WHEN Column_To_Test = 'Authorization of COA' THEN t.date_added
ELSE null
END AS date_added
FROM request r
LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
WHERE r.client_id='$cid'
AND r.created_at BETWEEN '$start' AND '$end'
AND a_s.department_id = '$dept'
GROUP BY r.request_id
ORDER BY `r`.`created_at` DESC "
This way the solution is more portable between the databases. Mysql offers if() function, ms sql iif() to achieve the above, but those are product specific solutions.

Answer 3

Try to use the Case statements like.
CASE WHEN ( Condition_1 AND Condition_2 OR ... Condition_N) THEN (Value_1) ELSE (Value_1) END
Here Condition_1 can be column record value and its combination of Column record values
and Value_1 can be single value [Ex: Null or "some string"] or Returned value from nested sql statement like [Ex: (select count(*) from ...)]

Answer 4

Give this a try:
SELECT r.request_id, r.product_name, r.created_at, r.can,a_s.stat, r.client_id, CONCAT(u.fname,' ',u.lname) 'analyst' ,
IFF(nameOfColumnToTest = 'Authorization of COA', t.date_added, null) AS DateAdded
FROM request r
LEFT OUTER JOIN assigned_samples a_s ON r.request_id = a_s.labref
LEFT OUTER JOIN user u ON a_s.analyst_id = u.id
LEFT OUTER JOIN tracking_table t ON r.request_id = t.labref
WHERE r.client_id='$cid'
AND r.created_at BETWEEN '$start' AND '$end'
AND a_s.department_id = '$dept'
GROUP BY r.request_id
ORDER BY `r`.`created_at` DESC "