Explain me ways to understand this complex query - mysql

Question

Can anyone help me to understand this big sql query. How do I break it down in small chunks to understand it ?
select t.Instrument as Instrument ,ClearingId as ClearingId,
ISNULL( PrevQty ,0) AS PrevQty,SettlePrice,
ISNULL(TodayBuyQty,0) as TodayBuyQty,
ISNULL( TodaySellQty ,0) AS TodaySellQty,
ISNULL(PrevQty +TodayBuyQty-TodaySellQty,0) as NetQty,
TodayBuyPrice, TodaySellPrice,LTP,PnL,Token
from
(
select Instrument,w.ClearingId as ClearingId,
ISNULL( PrevQty ,0) AS PrevQty,ISNULL(TodayBuyQty,0) as TodayBuyQty,
ISNULL( TodaySellQty ,0) AS TodaySellQty,
TodayAvgBuyPrice as TodayBuyPrice,TodayAvgSellPrice as TodaySellPrice,
LTP,PnL,w.Token
from
(
select Symbol as Instrument, ClearingId,
ISNULL(TodayBuyQty,0) as TodayBuyQty,
TodayAvgBuyPrice,
ISNULL( -TodaySellQty ,0) AS TodaySellQty,
TodayAvgSellPrice, NULL as LTP ,NULL as PnL ,
w.Token as Token
from
(
select Token, sum(Qty) as NetPositionQty, ClearingId,
sum(CASE WHEN Qty < 0 THEN Qty ELSE 0 END) as TodaySellQty,
sum(CASE WHEN Qty > 0 THEN Qty ELSE 0 END) as TodayBuyQty,
sum(CASE WHEN Qty < 0 THEN Qty * Price ELSE 0 END)
/
NULLIF(sum(CASE WHEN Qty < 0 THEN Qty ELSE 0 END), 0) as TodayAvgSellPrice,
sum(CASE WHEN Qty > 0 THEN Qty * Price ELSE 0 END)
/
NULLIF(sum(CASE WHEN Qty > 0 THEN Qty ELSE 0 END), 0) as TodayAvgBuyPrice
from
(
select m.Token,
(CASE WHEN ClearingId = 'SATP' THEN 'STRAITS' ELSE CASE WHEN ClearingId = 'BATP' THEN 'BPI' ELSE 'UOB' END END ) as ClearingId ,
Price/CAST(Multiplier AS float ) as Price,Qty
from
(
select Token , Exchange as ClearingId ,
LastTradePrice as Price ,
CASE WHEN Side = 'S' THEN -LastTradeQuantity ELSE LastTradeQuantity END as Qty
from Strategy_Orders
where ExchangeStatus in (9,10) )m
JOIN TokenMap t ON ( m.Token = t.Token)
UNION ALL
select m.Token, (CASE WHEN ClearingId = 'SATP' THEN 'STRAITS' ELSE CASE WHEN ClearingId = 'BATP' THEN 'BPI' ELSE 'UOB' END END ) as ClearingId ,
Price/CAST(Multiplier AS float ) as Price,
Qty
from
(
select Token , Exchange as ClearingId ,
LastTradePrice as Price ,
CASE WHEN Side = 'S' THEN -LastTradeQuantity ELSE LastTradeQuantity END as Qty
from Manual_Orders
where ExchangeStatus in (9,10) )m
JOIN TokenMap t ON ( m.Token = t.Token)
UNION ALL
select Token , ClearingId , TodayBuyPrice ,
TodayBuyQty as Qty
from EOD_Holdings
where CurrentDate =
( select top 1 CurrentDAte from EOD_Holdings
order by CurrentDAte desc
)
UNION ALL
select Token , ClearingId , TodaySellPrice ,
TodaySellQty as Qty
from EOD_Holdings
where CurrentDate = (
select top 1 CurrentDAte from EOD_Holdings
order by CurrentDAte desc
)
) x
group by Token,ClearingId) w
JOIN(select Token, Symbol from TokenMAp ) h on w.Token = h.Token
) w
FULL OUTER JOIN(
select Token, PrevQty , ClearingId
from EOD_Holdings
where CurrentDate = ( select top 1 CurrentDAte from EOD_Holdings order by CurrentDAte desc
)
) h
on w.Token = h.Token and w.ClearingId = h.ClearingId
)t
JOIN (
select * from LatestSettlePrices
) sp
on t.Instrument = sp.Instrument

Answer 1

You can break the query into chunks by looking at each subquery ("select ..." ) separately and running them to see the results. You need to start with the innermost queries that do not have other select statements in the "from" or "where" clause.
Also note that this query does not seem to be a clear, neither an optimal solution.
You would want to avoid using full outer joins and union all statements for the best performance.