Extended Syntax

Similar to DolphinDB, Python Parser's SQL programming features distributed computing. The following table introduces SQL statements supported and unsupported by Python Parser. For details about each statement, please refer to SQL Reference.

Notes:

• To call DolphinDB's built-in functions, it is necessary to use toddb() to convert Python objects into DolphinDB objects.

• In Python Parser scripts other than SQL, pairs or sequences cannot be accessed through "a:b" or "a..b" and can only be accessed with functions pair or seq.

• Both SQL and DolphinDB use "a:b" to specify a range.

import dolphindb as ddb
// Create a database
dbName="dfs://test"
if existsDatabase(dbName):
	dropDatabase(dbName)

// To call DolphinDB's built-in functions, it is necessary to use toddb() to convert Python objects into DolphinDB objects
db1=database("", ddb.HASH, [ddb.SYMBOL, 10].toddb())
db2=database("", ddb.LIST, ["A"+string(seq(1,10)), "B"+string(seq(1,10)), "C"+string(seq(1,10))].toddb())
db=database(dbName, ddb.COMPO, [db1, db2].toddb())

// Create a table
// In Python Parser scripts other than SQL, pairs or sequences cannot be accessed through "a:b" or "a..b" and can only be accessed with functions pair or seq
n=20
id=symbol(string(rand(uuid(), n)))
sym=rand(flatten(("A"+string(seq(1,10)), "B"+string(seq(1,10)), "C"+string(seq(1,10))).toddb()), n)
qty=rand(100, n)
t=table(id, sym, qty)

pt=db.createPartitionedTable(t, `pt, [`id, `sym].toddb()).append!(t)

// Make a query
// Both SQL and DolphinDB use "a:b" to specify a range
select top 0:5 * from pt where sym like "A%"

DolphinDB supports metacode for dynamic expression generation. A metacode block contains objects or expressions enclosed by angle brackets <>.

Python Parser shares the same coding conventions with DolphinDB, but their syntax differs. For example, the current version of Python Parser does not support some of DolphinDB-specific syntax, such as "1 2 3", "a:b", "a..b", and "func [1,2,3]".

eval(<1 + 3>)

Output: 4

sqlColAlias(<avg(PRC)>, `avgPRC)

Output: < avg(PRC) as avgPRC >

n=20
id=symbol(string(rand(uuid(), n)))
sym=rand(flatten(("A"+string(seq(1,10)), "B"+string(seq(1,10)), "C"+string(seq(1,10))).toddb()), n)
qty=rand(100, n)
t=table(id, sym, qty)
sql(select=sqlCol("*"), from=t, groupBy=sqlCol(`sym), groupFlag=0, limit=1)

Output: < select top 1 * from tf0746a0500000000 context by sym >

Reference: Metaprogramming.

The timer(n): statement calculates the execution time of the specified Python Parser statements. The parameter n is the number of executions.

def fib(n):
    xs = []
    a, b = 0, 1
    while a < n:
        xs.append(a)
        a, b = b, a + b
	return xs
timer(1000): fib(2000)

Time elapsed: 108.709 ms

Reference: timer.

In Python Parser, strings can be created not only with single quotes (') or double quotes (") as in Python but also with back quotes (`) as in DolphinDB.