Convert CROSS APPLY in DB2 database - sql-server

Question

I am trying to convert the below MSSQL query into DB2 query.
But i am facing issues . I got to know "CROSS APPLY" doesnt exist for DB2
SQL Server query:
SELECT DISTINCT p.ID,
p.COMPANY,
p.NAME,
format(d.startTime, 'yyyy-MM-dd HH:mm:ss.fff')
FROM PROCESS p
CROSS APPLY (SELECT MAX(END_TIME) AS startTime FROM PROCESS WHERE ID = (SELECT MAX(ID) FROM PROCESS)) AS d
WHERE p.ID = (SELECT MAX(ID) FROM PROCESS)
Error:
Error: com.ibm.db2.jcc.c.SqlException: DB2 SQL error: SQLCODE: -104, SQLSTATE: 42601, SQLERRMC: APPLY;N process
CROSS;JOIN
How the above query can be converted into DB2 query format?

Answer 1

The SQL Server manual says that a CROSS APPLY is used with table functions.
https://learn.microsoft.com/en-us/sql/t-sql/queries/from-transact-sql?view=sql-server-ver15#using-apply
That the right_table_source can use a table-valued function that takes a column from the left_table_source as one of the arguments of the function.
Your example does not use any, so I assume it is simply the equivalent to a CROSS JOIN in Db2.
By the way, this statement would likely get the same result (assuming COMPANY and NAME are the same for a given ID)
SELECT
ID
, COMPANY
, NAME
, format(END_TIME, 'yyyy-MM-dd HH:mm:ss.fff')
FROM
( SELECT *
, ROW_NUMBER() OVER(ORDER BY ID DESC, END_TIME DESC) AS RN
FROM
PROCESS p
)
WHERE
RN = 1
This might or might not be more optimal at execution time

Answer 2

The DB2 equivalent is the terribly named TABLE operator. The name makes it very challenging to find documentation.
See if this works for your query.
SELECT DISTINCT p.ID,
p.COMPANY,
p.NAME
FROM PROCESS p
JOIN TABLE (
SELECT ID
,MAX(END_TIME) AS startTime
FROM PROCESS
WHERE ID = (
SELECT MAX(ID)
FROM PROCESS
)
GROUP BY ID
) AS D
ON P.ID = D.ID
However, I've found CTEs in DB2 to be very effective. In SQL Server, a CTE is almost like a layer applied to a query, similar to a view. The expressions in the CTE are generally combined with the underlying statements and executed as a single statement.
I'm not a DB2 expert, but it seems to me that a CTE is materialized to an internal table and the result is combined with the remaining statements.
With the same query, data and indexes, DB2 and SQL Server can have very different performance when CTEs are involved.