MYSQL updating a table containing a join and subquery - mysql

Question

I am relatively new to SQL, i am trying to update monthly salary based on employees working for a certain duration, the query displays the data using info from the person and employee table but it won't update, i keep getting a 'operand should contain 1 column' error? How would i go about displaying all the data and be able to update the monthly_salary column as well? Thanks.
UPDATE employee ep set monthly_salary = monthly_salary*1.15 = all(
SELECT p.person_id, p.name_first, p.name_last, ep.monthly_salary, ep.start_date, curdate() as today_date,
TIMESTAMPDIFF(month,ep.start_date,curdate()) as duration_months
FROM employee ep
INNER JOIN person p ON ep.person_id = p.person_id having duration_months > 24);
query result
I want this expected result but the monthly salary hasn't been updated yet, is it possible to display this and update the monthly_salary?

Answer 1

You are not able to do both in a single query. Typically one would run a "select query" to inspect if the desired logic appears correct, e.g.
SELECT
p.person_id
, p.name_first
, p.name_last
, ep.start_date
, curdate() as today_date
, TIMESTAMPDIFF(month,ep.start_date,curdate()) as duration_months
FROM employee ep
INNER JOIN person p ON ep.person_id = p.person_id
WHERE ep.start_date < curdate() - INTERVAL 24 MONTH
;
In that query the important piece of logic is the where clause which seeks out any employees with a start date earlier than today - 24 months.
If that logic is correct, then apply the same logic in an "update query":
UPDATE employee ep
SET monthly_salary = monthly_salary*1.15
WHERE ep.start_date < curdate() - INTERVAL 24 MONTH
;
Syntax notes:
you cannot string multiple conditions together using multiple equality operators (monthly_salary = monthly_salary*1.15 = all(...) there are 2 = signs in that
x = all() requires that all values returned by a subquery will equal x
the having clause is NOT just a substitute for a where clause. A having clause is designed for evaluating aggregated data e.g. having count(*) > 2
Finally, while it was inventive to use the having clause, what you were doing was gaining access to the alias 'duration_months', so you could simply have done this instead:
where TIMESTAMPDIFF(month,ep.start_date,curdate()) > 24
BUT this is not a good way to filter information because it requires running a function on every row of data before a decision can be reached. This has he effect of making queries slower. Compare that to the following:
WHERE ep.start_date < curdate() - INTERVAL 24 MONTH
ep.start_date is not affected by any function, and curdate() - INTERVAL 24 MONTH is just one calculation (not done every row). So this is much more efficient (also known as "sargable").