使用联接表中的if条件确定列数据

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

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 "

Answer 3

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 4

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 ...)]