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select

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 ?, see Functional SQL

Syntax

Below, square brackets mark optional elements.

select [Lexp] [ps] [by pb] from texp [where pw]

qSQL syntax

The select query returns a table for both call-by-name and call-by-value.

Minimal form

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

Select phrase

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.

Limit expression

To limit the returned results you can include a limit expression Lexp

select[n]
select[m n]
select[order]
select[n;order]
select distinct

where

  • n limits the result to the first n rows of the selection if positive, or the last n rows if negative
  • m is the number of the first row to be returned: useful for stepping through query results one block of n at a time
  • order is a column (or table) and sort order: use < for ascending, > for descending
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.

Performance

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.

By phrase

A 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

The SQL GROUP BY statement

The 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.

Cond

Cond is not supported inside query templates: see qSQL.


delete, exec, update
qSQL, Functional SQL
Q for Mortals §9.3 The select Template