How to do self join on min/max - sql

Question

I am new to sql queries.
Table is defined as
( symbol varchar,
high int,
low int,
today date,
Primary key (symbol, today)
)
I need to find for each symbol in a given date range, max(high) and min(low) and corresponding dates for max(high) and min(low).
Okay to get first max date and min date in given table.
In a given date range some dates may be missing. If start date is not present then next date should be used and if last date is not present then earlier available date should be used
Data is for one year and around 5000 symbols.
I tried something like this
SELECT a.symbol,
a.maxValue,
a.maxdate,
b.minValue,
b.mindate
FROM (
SELECT table1.symbol, max_a.maxValue, max_a.maxdate
FROM table1
INNER JOIN (
SELECT table1.symbol,
max(table1.high) AS maxValue,
table1.TODAY AS maxdate
FROM table1
GROUP BY table1.symbol
) AS max_a
ON max_a.symbol = table1.symbol
AND table1.today = max_a.maxdate
) AS a
INNER JOIN (
SELECT symbol,
min_b.minValue,
min_b.mindate
FROM table1
INNER JOIN (
SELECT symbol,
min(low) AS minValue,
table1.TODAY AS mindate
FROM table1
GROUP BY testnsebav.symbol
) AS min_b
ON min_b.symbol = table1.symbol
AND table1.today = min_b.mindate
) AS b
ON a.symbol = b.symbol

Answer 1

The first INNER query pre-qualifies for each symbol what the low and high values are within the date range provided. After that, it joins back to the original table again (for same date range criteria), but also adds the qualifier that EITHER the low OR the high matches the MIN() or MAX() from the PreQuery. If so, allows it in the result set.
Now, the result columns. Not knowing which version SQL you were using, I have the first 3 columns as the "Final" values. The following 3 columns after that come from the record that qualified by EITHER of the qualifiers. As stocks go up and down all the time, its possible for the high and/or low values to occur more than once within the same time period. This will include ALL those entries that qualify the MIN() / MAX() criteria.
select
PreQuery.Symbol,
PreQuery.LowForSymbol,
PreQuery.HighForSymbol,
tFinal.Today as DateOfMatch,
tFinal.Low as DateMatchLow,
tFinal.High as DateMatchHigh
from
( select
t1.symbol,
min( t1.low ) as LowForSymbol,
max( t1.high ) as HighForSymbol
from
table1 t1
where
t1.today between YourFromDateParameter and YourToDateParameter
group by
t1.symbol ) PreQuery
JOIN table1 tFinal
on PreQuery.Symbol = tFinal.Symbol
AND tFinal.today between YourFromDateParameter and YourToDateParameter
AND ( tFinal.Low = LowForSymbol
OR tFinal.High = HighForSymbol )