MYSQL SELECT is slow when crossing multiple tables - mysql

Question

I have a mysql query which is to return the only 1 record that need to cross multiple table. However, the mysql query is slow when executing.
Query:
SELECT *,
(SELECT TreeName FROM sys_tree WHERE TreeId = Mktg_Unit_Booking.ProjectLevelId) AS PhaseName,
(CASE WHEN ProductType = 'U' THEN (SELECT UnitNo FROM prop_unit pu WHERE pu.UnitId = mktg_unit_booking.UnitId)
ELSE (SELECT BayNo FROM prop_car_park pcp WHERE pcp.CarParkId = UnitId) END) AS UnitNo,
(SELECT CustomerName FROM mktg_customer mc WHERE mc.CustomerId = mktg_unit_booking.CustomerId) AS CustomerName
FROM Mktg_Unit_Booking
WHERE IsDeleted <> '1' AND IsApproved = '1'
AND UnitId = 1110 AND ProductType = 'U'
ORDER BY UnitNo
I have run EXPLAIN in the query and I got this:
Any other suggestion on how to improve the speed of the query?
Thank you!

Answer 1

you are doing the cross product, instead of that you should use join.

Answer 2

Don't use sub-queries in select statement instead use proper join on Mktg_Unit_Booking in after from statement.
you query should something look like :
select
sys_tree.TreeName AS PhaseName,
case
WHEN Mktg_Unit_Booking.ProductType = 'U' then prop_unit.UnitNo
else prop_car_park.BayNo
end as UnitNo,
mktg_customer.CustomerName AS CustomerName
FROM Mktg_Unit_Booking
left join sys_tree on sys_tree.TreeId = Mktg_Unit_Booking.ProjectLevelId
left join prop_unit on prop_unit.UnitId = Mktg_Unit_Booking.UnitId
left join prop_car_park on prop_car_park.CarParkId = Mktg_Unit_Booking.UnitId
left join mktg_customer on mktg_customer.CustomerId = Mktg_Unit_Booking.CustomerId
WHERE IsDeleted <> '1' AND IsApproved = '1'
AND UnitId = 1110 AND ProductType = 'U'
ORDER BY UnitNo;
I have assumed that each table consists of only 1 matching tuple. If there are more then your logic needs to be modified.