Db2 for i - dynamic FROM clause - sql

Question

Environment: Db2 for i, version 7.3
Library/table structure:
CORPORATE/TENANTS
LIB01/INVOICE
LIB02/INVOICE
LIB03/INVOICE
…
LIBxx/INVOICE
The CORPORATE/TENANTS table contains a list of libraries where information about each tenant is stored. It has this structure and data:
CREATE OR REPLACE TABLE TENANTS (
ID BIGINT GENERATED ALWAYS AS IDENTITY (START WITH 1),
TENANT CHAR(10) NOT NULL,
PRIMARY KEY(ID)
) RCDFMT TENANTSR;
RUNSQLSTM SRCFILE(HILLB/QDDLSRC) SRCMBR(TENANTS) DFTRDBCOL(CORPORATE)
+--+------+
|ID|TENANT|
+--+------+
| 1|LIB01 |
| 2|LIB02 |
|..|......|
|99|LIB99 |
+--+------+
The LIBxx/INVOICE tables are all identical to each other and have this structure:
CREATE OR REPLACE TABLE INVOICE (
ID BIGINT GENERATED ALWAYS AS IDENTITY (START WITH 1),
PAYDAT INTEGER(6,0) NOT NULL,
AMOUNT DECIMAL(15,2) NOT NULL DEFAULT 0,
PRIMARY KEY(ID)
) RCDFMT INVOICER;
+--+------+------+
|ID|PAYDAT|AMOUNT|
+--+------+------+
| 1|180701|100.00|
| 2|180801| 35.00|
|..|......|......|
+--+------+------+
I want to generate a list of invoice amounts for all tenants for a given date:
180701 LIB01 100.00
180701 LIB02 140.00
180701 LIB03 74.00
…
Conceptually what I want to do is this (yes, I know this is invalid SQL):
SELECT PAYDAT, TENANT, AMOUNT
FROM $X.INVOICE
WHERE PAYDAT = 180701;
I want to pull data from the INVOICE table for each TENANT but I know the FROM clause cannot be dynamic like this. I’m sure this kind of query has a name but I don’t know what it is so I’m unable to effectively use a search engine to find what I need.
This would be trivial to solve with an RPGLE program but I need a pure SQL solution.
Please note - the LIBxx values CANNOT be hardcoded in any way. These values can change at any time.

Answer 1

To do what you want, you can use a stored procedure with an EXECUTE IMMEDIATE in a loop to build a result set. Something like this:
Note: this is not a complete cut and paste solution, but you can modify it to do what you want.
CREATE OR REPLACE PROCEDURE GETINVOICEAMOUNTS ( )
DYNAMIC RESULT SETS 1
LANGUAGE SQL
NOT DETERMINISTIC
MODIFIES SQL DATA
CALLED ON NULL INPUT
SET OPTION COMMIT = *NONE
BEGIN
DECLARE STMT VARCHAR(1024);
DECLARE RECORD_FOUND INTEGER DEFAULT 1;
DECLARE LIBRARY CHAR(10);
DECLARE C1 CURSOR FOR
SELECT TENANT FROM CORPORATE/TENANT;
DECLARE C2 CURSOR WITH RETURN FOR
SELECT * FROM SESSION.TMP ;
DECLARE CONTINUE HANDLER FOR NOT FOUND
SET RECORD_FOUND = 0;
DECLARE GLOBAL TEMPORARY TABLE TMP
(PAYDAT INTEGER(6,0),
TENANT CHAR(10),
AMOUNT DECIMAL(15,2))
WITH REPLACE;
OPEN C1;
LOOP
FETCH C1 INTO LIBRARY;
IF RECORD_FOUND = 0;
LEAVE LOOP;
END IF;
SET STMT = 'INSERT INTO SESSION.TMP SELECT PAYDAT, LIBRARY, AMOUNT FROM ' || RTRIM(LIBRARY) || '.INVOICE WHERE PAYDAT = 180701';
EXECUTE IMMEDIATE STMT
END LOOP;
CLOSE C1;
OPEN C2;
END;
I gave you more than I planned to. But, one specific modification you will invariably need is to parameterize the date that you want to retrieve.
This is how it works:
A global temporary table named TMP is used to collect the records to be returned in a result set. Once all the records are collected, a cursor is opened over TMP and the procedure ends. This causes the values collected in TMP to be returned as a result set.
To collect the values the CORPORATE/TENANT file is read, and the column TENANT is retrieved into the variable LIBRARY. For each record a statement is built that concatenates LIBRARY into an INSERT statement. This statement is executed which loads the record into TMP. I am using EXECUTE IMMEDIATE because I cannot use a parameter marker to replace the table reference in the INSERT statement, so a prepared statement is just extra work.

Answer 2

You could use UNION ALL:
SELECT sub.PAYDAT, t.TENANT, sub.AMOUNT
FROM (SELECT * FROM LIB01.INVOICE
UNION ALL
SELECT * FROM LIB02/INVOICE
...
SELECT * FROM LIB0n/INVOICE) sub
JOIN TENANTS t
ON sub.id = t.id
WHERE SUB.PAYDAT = 180701;
It is an SELECT * FROM sales + #yymm template.
EDIT:
More secure way is to create a view:
CREATE VIEW combined_invoice
AS
SELECT * FROM LIB01.INVOICE
UNION ALL
SELECT * FROM LIB02/INVOICE
...
SELECT * FROM LIB0n/INVOICE;
And query:
SELECT sub.PAYDAT, t.TENANT, sub.AMOUNT
FROM combined_invoice sub
JOIN TENANTS t
ON sub.id = t.id
WHERE SUB.PAYDAT = 180701;
Of course view should be altered after adding/removing tables.