Select all or part of a table, possibly with new columns
select is a qSQL query template and varies from regular q syntax.
For the Select operator
Below, square brackets mark optional elements.
select [Lexp] [ps] [by pb] from texp [where pw]
select query returns a table for both call-by-name and call-by-value.
The minimal form of the query returns the evaluated table expression.
q)tbl:( id:1 1 2 2 2;val:100 200 300 400 500) q)select from tbl id val ------ 1 100 1 200 2 300 2 400 2 500
The Select phrase specifies the columns of the result table, one per subphrase.
Absent a Select phrase, all the columns of the table expression are returned.
(Unlike SQL, no
* wildcard is required.)
q)t:( c1:`a`b`c; c2:10 20 30; c3:1.1 2.2 3.3) q)select c3, c1 from t c3 c1 ------ 1.1 a 2.2 b 3.3 c q)select from t c1 c2 c3 --------- a 10 1.1 b 20 2.2 c 30 3.3
A computed column in the Select phrase cannot be referred to in another subphrase.
To limit the returned results you can include a limit expression Lexp
select[n] select[m n] select[order] select[n;order] select distinct
nlimits the result to the first
nrows of the selection if positive, or the last
nrows if negative
mis the number of the first row to be returned: useful for stepping through query results one block of
nat a time
orderis a column (or table) and sort order: use
select[3;>price] from bids where sym=s,size>0
This would return the three best prices for symbol
s with a size greater than 0.
This construct works on in-memory tables but not on memory-mapped tables loaded from splayed or partitioned files.
select[n] applies the Where phrase on all rows of the table, and takes the first
n rows, before applying the Select phrase.
So if you are paging it is better to store the result of the query somewhere and
select[n,m] from there, rather than run the filter again.
select distinct returns only unique records in the result.
select query that includes a By phrase returns a keyed table.
The key columns are those in the By phrase; values from other columns are grouped, i.e. nested.
q)k:`a`b`a`b`c q)v:10 20 30 40 50 q)select c2 by c1 from (c1:k;c2:v) c1| c2 --| ----- a | 10 30 b | 20 40 c | ,50 q)v group k / compare the group keyword a| 10 30 b| 20 40 c| ,50
Unlike in SQL, columns in the By phrase
- are included in the result and need not be specified in the Select phrase
- can include computed columns
ungroup keyword reverses the grouping, though the original order is lost.
q)ungroup select c2 by c1 from (c1:k;c2:v) c1 c2 ----- a 10 a 30 b 20 b 40 c 50
q)t:( name:`tom`dick`harry`jack`jill;sex:`m`m`m`m`f;eye:`blue`green`blue`blue`gray) q)t name sex eye --------------- tom m blue dick m green harry m blue jack m blue jill f gray q)select name,eye by sex from t sex| name eye ---| ------------------------------------------ f | ,`jill ,`gray m | `tom`dick`harry`jack `blue`green`blue`blue q)select name by sex,eye from t sex eye | name ---------| --------------- f gray | ,`jill m blue | `tom`harry`jack m green| ,`dick
A By phrase with no Select phrase returns the last row in each group.
q)select by sex from t sex| name eye ---| --------- f | jill gray m | jack blue
Where there is a By phrase, and no sort order is specified, the result is sorted ascending by its key.