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QSQL query templates

delete delete rows or columns from a table exec return columns from a table, possibly with new columns select return part of a table, possibly with new columns update add rows or columns to a table

The query templates of qSQL share a query syntax that varies from the syntax of q and closely resembles conventional SQL. For many use cases involving ordered data it is significantly more expressive.

Structured Query Language

Template syntax

Below, square brackets mark optional elements; a slash begins a trailing comment.

select [Lexp] [ps] [by pb] from texp [where pw] exec [distinct] [ps] [by pb] from texp [where pw] update ps [by pb] from texp [where pw] delete from texp [where pw] / rows delete ps from texp / columns

A template is evaluated in the following order.

From phrase texp Where phrase pw By phrase pb Select phrase ps Limit expression Lexp

From phrase

The From phrase from texp is required in all query templates.

The table expression texp is

  • a table or dictionary (call-by-value)
  • the name of a table or dictionary, in memory or on disk, as a symbol atom (call-by-name)

Examples:

update c:b*2 from ([]a:1 2;b:3 4)   / call by value
select a,b from t                   / call by value
select a,b from `t                  / call by name
update c:b*2 from `:path/to/db      / call by name

Limit expressions

Limit expressions restrict the results returned by select or exec. (For exec there is only one: distinct). They are described in the articles for select and exec.

Result and side effects

In a select query, the result is a table or dictionary.

In an exec query the result is a list of column values, or dictionary.

In an update or delete query, where the table expression is a call

  • by value, the query returns the modified table or a dictionary
  • by name, the table or dictionary is amended in place (in memory or on disk) as a side effect, and its name returned as the result
q)t1:t2:([]a:1 2;b:3 4)

q)update a:neg a from t1
a  b
----
-1 3
-2 4
q)t1~t2   / t1 unchanged
1b

q)update a:neg a from `t1
`t1
q)t1~t2   / t1 changed
0b

Phrases and subphrases

ps, pb, and pw are respectively the Select, By, and Where phrases. Each phrase is a comma-separated list of subphrases.

A subphrase is a q expression in which names are resolved with respect to texp and any table/s linked by foreign keys. Subphrases are evaluated in order from the left, but each subphrase expression is evaluated right-to-left in normal q syntax.

To use the Join operator within a subphrase, parenthesize the subphrase.


q)select (id,'4),val from tbl
x   val
-------
1 4 100
1 4 200
2 4 300
2 4 400
2 4 500

Names in subphrases

A name in a subphrase is resolved (in order) as the name of

  1. column or key name
  2. local name in (or argument of) the encapsulating function
  3. global name in the current working namespace – not necessarily the space in which the function was defined

Dot notation allows you to refer to foreign keys.

Suppliers and parts database sp.q

q)\l sp.q
+`p`city!(`p$`p1`p2`p3`p4`p5`p6`p1`p2;`london`london`london`london`london`lon..
(`s#+(,`color)!,`s#`blue`green`red)!+(,`qty)!,900 1000 1200
+`s`p`qty!(`s$`s1`s1`s1`s2`s3`s4;`p$`p1`p4`p6`p2`p2`p4;300 200 100 400 200 300)

q)select sname:s.name, qty from sp
sname qty
---------
smith 300
smith 200
smith 400
smith 200
clark 100
smith 100
jones 300
jones 400
blake 200
clark 200
clark 300
smith 400

Implicit joins

You can refer explicitly to namespaces.


select (`. `toplevel) x from t

Duplicate names for columns or groups

select auto-aliases colliding duplicate column names for either select az,a from t, or select a by c,c from t, but not for select a,a by a from t.

Such a collision throws a 'dup names for cols/groups a error during parse, indicating the first column name which collides. (Since V4.0 2020.03.17.)


q)parse"select b by b from t"
'dup names for cols/groups b
  [2]  select b by b from t
       ^

The easiest way to resolve this conflict is to explicitly rename columns. e.g. select a,b by c:a from t.

When compiling functions, the implicit args x, y, z are visible to the compiler only when they are not inside the Select, By, and Where phrases. The table expression is not masked. This can be observed by taking the value of the function and observing the second item: the args.

q)args:{(value x)1}
q)args{} / no explicit args, so x is a default implicit arg of identity (::)
,`x

q)/from phrase is not masked, y is detected as an implicit arg here
q)args{select from y where a=x,b=z}
`x`y
q)args{[x;y;z]select from y where a=x,b=z} / x,y,z are now explicit args
`x`y`z

q)/call with wrong number of args results in rank error
q){select from ([]a:0 1;b:2 3) where a=x,b=y}[0;2]
'rank
  [0]  {select from ([]a:0 1;b:2 3) where a=x,b=y}[0;2]
       ^

q)/works with explicit args
q){[x;y]select from ([]a:0 1;b:2 3) where a=x,b=y}[0;2]
a b
---
0 2

Computed columns

In a subphrase, a q expression computes a new column or key, and a colon names it.

q)t:([] c1:`a`b`c; c2:10 20 30; c3:1.1 2.2 3.3)

q)select c1, c3*2 from t
c1 c3
------
a  2.2
b  4.4
c  6.6

q)select c1, dbl:c3*2 from t
c1 dbl
------
a  2.2
b  4.4
c  6.6

In the context of a query, the colon names a result column or key. It does not assign a variable in the workspace.

If a computed column or key is not named, q names it if possible as the leftmost term in the column expression, else as x. If a computed name is already in use, q suffixes it with 1, 2, and so on as needed to make it unique.

q)select c1, c1, 2*c2, c2+c3, string c3 from t
c1 c11 x  c2   c3
--------------------
a  a   20 11.1 "1.1"
b  b   40 22.2 "2.2"
c  c   60 33.3 "3.3"

Virtual column i

A virtual column i represents the index of each record, i.e., the row number.

Partitioned tables

In a partitioned table i is the index (row number) relative to the partition, not the whole table.

Because it is implicit in every table, it never appears as a column or key name in the result.

q)select i, c1 from t
x c1
----
0 a
1 b
2 c

q)select from t where i in 0 2
c1 c2 c3
---------
a  10 1.1
c  30 3.3

Where phrase

The Where phrase with a boolean list selects records.

q)select from t where 101b
c1 c2 c3
---------
a  10 1.1
c  30 3.3

Subphrases specify successive filters.

q)select from t where c2>15,c3<3.0
c1 c2 c3
---------
b  20 2.2

q)select from t where (c2>15) and c3<3.0
c1 c2 c3
---------
a  10 1.1
b  20 2.2
c  30 3.3

The examples above return the same result but have different performance characteristics.

In the second example, all c2 values are compared to 15, and all c3 values are compared to 3.0. The two result vectors are ANDed together.

In the first example, only c3 values corresponding to c2 values greater than 15 are tested.

Efficient Where phrases start with their most stringent tests.

Querying a partitioned table

When querying a partitioned table, the first Where subphrase should select from the value/s used to partition the table.

Otherwise, kdb+ will (attempt to) load into memory all partitions for the column/s in the first subphrase.

Use fby to filter on groups.

Sorting

Unlike SQL, the query templates make no provision for sorting. Instead use xasc and xdesc to sort the query results.

As the sorts are stable, they can be combined for mixed sorts.

q)sp
s  p  qty
---------
s1 p1 300
s1 p2 200
s1 p3 400
s1 p4 200
s4 p5 100
s1 p6 100
s2 p1 300
s2 p2 400
s3 p2 200
s4 p2 200
s4 p4 300
s1 p5 400

q)`p xasc `qty xdesc select from sp where p in `p2`p4`p5
s  p  qty
---------
s2 p2 400
s1 p2 200
s3 p2 200
s4 p2 200
s4 p4 300
s1 p4 200
s1 p5 400
s4 p5 100

Performance

  • Select only the columns you will use.
  • Use the most restrictive constraint first.
  • Ensure you have a suitable attribute on the first non-virtual constraint (e.g.`p or `g on sym).
  • Constraints should have the unmodified column name on the left of the constraint operator (e.g. where sym in syms,…)
  • When aggregating, use the virtual field first in the By phrase. (E.g. select .. by date,sym from …)

Tip

…where `g=,`s within …
Maybe rare to get much speedup, but if the `g goes to 100,000 and then `s is 1 hour of 24 you might see some overall improvement (with overall table of 30 million).

Q for Mortals §14.3.6 Query Execution on Partitioned Tables

Multithreading

The following pattern will make use of secondary threads via peach

select … by sym, … from t where sym in …, … 

when sym has a `g or `p attribute. (Since V3.2 2014.05.02)

It uses peach for both in-memory and on-disk tables. For single-threaded, this is approx 6× faster in memory, 2× faster on disk, and uses less memory than previous releases – but mileage will vary. This is also applicable for partitioned DBs as

select … by sym, … from t where date …, sym in …, …

Table counts in a partitioned database

Special functions

The following functions (essentially .Q.a0 in q.k) receive special treatment within select:

avg     first   prd  
cor     last    sum 
count   max     var  
cov     med     wavg
dev     min     wsum

When used explicitly, such that it can recognize the usage, q will perform additional steps, such as enlisting results or aggregating across partitions. However, when wrapped inside another function, q does not know that it needs to perform these additional steps, and it is then left to the programmer to insert them.

q)select sum a from ([]a:1 2 3)
a
-
6
q)select {(),sum x}a from ([]a:1 2 3)
a
-
6

Cond

Cond is not supported inside qSQL expressions.

q)u:([]a:raze ("ref/";"kb/"),\:/:"abc"; b:til 6)
q)select from u where a like $[1b;"ref/*";"kb/*"]
'rank
  [0]  select from u where a like $[1b;"ref/*";"kb/*"]
                                  ^

Enclose in a lambda

q)select from u where a like {$[x;"ref/*";"kb/*"]}1b
a       b
---------
"ref/a" 0
"ref/b" 2
"ref/c" 4

or use the Vector Conditional instead.

Functional SQL

The interpreter translates the query templates into functional SQL for evaluation. The functional forms are more general, and some complex queries require their use. But the query templates are powerful, readable, and there is no performance penalty for using them.

Wherever possible, prefer the query templates to functional forms.

Stored procedures

Q for Mortals §9.9.10 Parameterized Queries

Views

Views
Q for Mortals §9.9.11 Views


fby, insert, upsert,
Functional SQL
Q for Mortals §9.0 Queries: q-sql