Q Language - Table
Tables are at the heart of kdb+. A table is a collection of named columns implemented as a dictionary. q tables are column-oriented.
Creating Tables
Tables are created using the following syntax −
q)trade:([]time:();sym:();price:();size:())
q)trade
time sym price size
-------------------
In the above example, we have not specified the type of each column. This will be set by the first insert into the table.
Another way, we can specify column type on initialization −
q)trade:([]time:`time$();sym:`$();price:`float$();size:`int$())
Or we can also define non-empty tables −
q)trade:([]sym:(`a`b);price:(1 2))
q)trade
sym price
-------------
a 1
b 2
If there are no columns within the square brackets as in the examples above, the table is unkeyed.
To create a keyed table, we insert the column(s) for the key in the square brackets.
q)trade:([sym:`$()]time:`time$();price:`float$();size:`int$())
q)trade
sym | time price size
----- | ---------------
One can also define the column types by setting the values to be null lists of various types −
q)trade:([]time:0#0Nt;sym:0#`;price:0#0n;size:0#0N)
Getting Table Information
Let’s create a trade table −
trade: ([]sym:`ibm`msft`apple`samsung;mcap:2000 4000 9000 6000;ex:`nasdaq`nasdaq`DAX`Dow)
q)cols trade / column names of a table
`sym`mcap`ex
q)trade.sym / Retrieves the value of column sym
`ibm`msft`apple`samsung
q)show meta trade / Get the meta data of a table trade.
c | t f a
----- | -----
Sym | s
Mcap | j
ex | s
Primary Keys and Keyed Tables
Keyed Table
A keyed table is a dictionary that maps each row in a table of unique keys to a corresponding row in a table of values. Let us take an example −
val:flip `name`id!(`John`Jenny`Jonathan;9 18 27)
/ a flip dictionary create table val
id:flip (enlist `eid)!enlist 99 198 297
/ flip dictionary, having single column eid
Now create a simple keyed table containing eid as key,
q)valid: id ! val
q)valid / table name valid, having key as eid
eid | name id
------| ---------------
99 | John 9
198 | Jenny 18
297 | Jonathan 27
ForeignKeys
A foreign key defines a mapping from the rows of the table in which it is defined to the rows of the table with the corresponding primary key.
Foreign keys provide referential integrity. In other words, an attempt to insert a foreign key value that is not in the primary key will fail.
Consider the following examples. In the first example, we will define a foreign key explicitly on initialization. In the second example, we will use foreign key chasing which does not assume any prior relationship between the two tables.
Example 1 − Define foreign key on initialization
q)sector:([sym:`SAMSUNG`HSBC`JPMC`APPLE]ex:`N`CME`DAQ`N;MC:1000 2000 3000 4000)
q)tab:([]sym:`sector$`HSBC`APPLE`APPLE`APPLE`HSBC`JPMC;price:6?9f)
q)show meta tab
c | t f a
------ | ----------
sym | s sector
price | f
q)show select from tab where sym.ex=`N
sym price
----------------
APPLE 4.65382
APPLE 4.643817
APPLE 3.659978
Example 2 − no pre-defined relationship between tables
sector: ([symb:`IBM`MSFT`HSBC]ex:`N`CME`N;MC:1000 2000 3000)
tab:([]sym:`IBM`MSFT`MSFT`HSBC`HSBC;price:5?9f)
To use foreign key chasing, we must create a table to key into sector.
q)show update mc:(sector([]symb:sym))[`MC] from tab
sym price mc
--------------------------
IBM 7.065297 1000
MSFT 4.812387 2000
MSFT 6.400545 2000
HSBC 3.704373 3000
HSBC 4.438651 3000
General notation for a predefined foreign key −
select a.b from c where a is the foreign key (sym), b is a
field in the primary key table (ind), c is the
foreign key table (trade)
Manipulating Tables
Let’s create one trade table and check the result of different table expression −
q)trade:([]sym:5?`ibm`msft`hsbc`samsung;price:5?(303.00*3+1);size:5?(900*5);time:5?(.z.T-365))
q)trade
sym price size time
-----------------------------------------
msft 743.8592 3162 02:32:17.036
msft 641.7307 2917 01:44:56.936
hsbc 838.2311 1492 00:25:23.210
samsung 278.3498 1983 00:29:38.945
ibm 838.6471 4006 07:24:26.842
Let us now take a look at the statements that are used to manipulate tables using q language.
Select
The syntax to use a Select statement is as follows −
select [columns] [by columns] from table [where clause]
Let us now take an example to demonstrate how to use Select statement −
q)/ select expression example
q)select sym,price,size by time from trade where size > 2000
time | sym price size
------------- | -----------------------
01:44:56.936 | msft 641.7307 2917
02:32:17.036 | msft 743.8592 3162
07:24:26.842 | ibm 838.6471 4006
Insert
The syntax to use an Insert statement is as follows −
`tablename insert (values)
Insert[`tablename; values]
Let us now take an example to demonstrate how to use Insert statement −
q)/ Insert expression example
q)`trade insert (`hsbc`apple;302.0 730.40;3020 3012;09:30:17.00409:15:00.000)
5 6
q)trade
sym price size time
------------------------------------------
msft 743.8592 3162 02:32:17.036
msft 641.7307 2917 01:44:56.936
hsbc 838.2311 1492 00:25:23.210
samsung 278.3498 1983 00:29:38.945
ibm 838.6471 4006 07:24:26.842
hsbc 302 3020 09:30:17.004
apple 730.4 3012 09:15:00.000
q)/Insert another value
q)insert[`trade;(`samsung;302.0; 3333;10:30:00.000]
']
q)insert[`trade;(`samsung;302.0; 3333;10:30:00.000)]
,7
q)trade
sym price size time
----------------------------------------
msft 743.8592 3162 02:32:17.036
msft 641.7307 2917 01:44:56.936
hsbc 838.2311 1492 00:25:23.210
samsung 278.3498 1983 00:29:38.945
ibm 838.6471 4006 07:24:26.842
hsbc 302 3020 09:30:17.004
apple 730.4 3012 09:15:00.000
samsung 302 3333 10:30:00.000
Delete
The syntax to use a Delete statement is as follows −
delete columns from table
delete from table where clause
Let us now take an example to demonstrate how to use Delete statement −
q)/Delete expression example
q)delete price from trade
sym size time
-------------------------------
msft 3162 02:32:17.036
msft 2917 01:44:56.936
hsbc 1492 00:25:23.210
samsung 1983 00:29:38.945
ibm 4006 07:24:26.842
hsbc 3020 09:30:17.004
apple 3012 09:15:00.000
samsung 3333 10:30:00.000
q)delete from trade where price > 3000
sym price size time
-------------------------------------------
msft 743.8592 3162 02:32:17.036
msft 641.7307 2917 01:44:56.936
hsbc 838.2311 1492 00:25:23.210
samsung 278.3498 1983 00:29:38.945
ibm 838.6471 4006 07:24:26.842
hsbc 302 3020 09:30:17.004
apple 730.4 3012 09:15:00.000
samsung 302 3333 10:30:00.000
q)delete from trade where price > 500
sym price size time
-----------------------------------------
samsung 278.3498 1983 00:29:38.945
hsbc 302 3020 09:30:17.004
samsung 302 3333 10:30:00.000
Update
The syntax to use an Update statement is as follows −
update column: newValue from table where ….
Use the following syntax to update the format/datatype of a column using the cast function −
update column:newValue from `table where …
Let us now take an example to demonstrate how to use Update statement −
q)/Update expression example
q)update size:9000 from trade where price > 600
sym price size time
------------------------------------------
msft 743.8592 9000 02:32:17.036
msft 641.7307 9000 01:44:56.936
hsbc 838.2311 9000 00:25:23.210
samsung 278.3498 1983 00:29:38.945
ibm 838.6471 9000 07:24:26.842
hsbc 302 3020 09:30:17.004
apple 730.4 9000 09:15:00.000
samsung 302 3333 10:30:00.000
q)/Update the datatype of a column using the cast function
q)meta trade
c | t f a
----- | --------
sym | s
price| f
size | j
time | t
q)update size:`float$size from trade
sym price size time
------------------------------------------
msft 743.8592 3162 02:32:17.036
msft 641.7307 2917 01:44:56.936
hsbc 838.2311 1492 00:25:23.210
samsung 278.3498 1983 00:29:38.945
ibm 838.6471 4006 07:24:26.842
hsbc 302 3020 09:30:17.004
apple 730.4 3012 09:15:00.000
samsung 302 3333 10:30:00.000
q)/ Above statement will not update the size column datatype permanently
q)meta trade
c | t f a
------ | --------
sym | s
price | f
size | j
time | t
q)/to make changes in the trade table permanently, we have do
q)update size:`float$size from `trade
`trade
q)meta trade
c | t f a
------ | --------
sym | s
price | f
size | f
time | t
