What is the best way to join tables - sql

Question

this is more like a general question.
I am looking for the best way to join 4, maybe 5 different tables. I am trying to create a Power Bi pulling live information from an IBM AS400 where customer service can type one of our parts number,
see how many parts we have in inventory, if none, see the lead time and if there are any orders already already entered for the typed part number.
SERI is our inventory table with 37180 records.
(active inventory that is available)
METHDM is our kit table with 37459 records.
(this table contains the bill of materials for custom kits, KIT A123 contains different part numbers in it witch are in SERI as well.)
STKA is our part lead time table with 76796 records.
(lead time means how long will it take for parts to come in)
OCRI is our sales order table with 6497 records.
(This table contains all customer orders)
I have some knowledge in writing queries but this one is more challenging of what I have created in the past. Should I start with the table that has the most records and start left joining the rest ?
From STKA 76796 records
Left join METHDM 37459 records on STKA
left join SERI 37180 records on STKA
left join OCRI 6497 records on STAK
Select
STKA.v6part as part,
STKA.v6plnt as plant,
STKA.v6tdys as pur_leadtime,
STKA.v6prpt as Pur_PrepLeadtime,
STKA.v6lead as Mfg_leadtime,
STKA.v6prpt as Mfg_PrepLeadTime,
METHDM.AQMTLP AS COMPONENT,
METHDM.AQQPPC AS QTYNEEDED,
SERI.HTLOTN AS BATCH,
SERI.HTUNIT AS UOM,
(HTQTY - HTQTYC) as ONHAND,
OCRI.DDORD# AS SALESORDER,
OCRI.DDRDAT AS PROMISED
from stka
left join METHDM on STKA.V6PART = METHDM.AQPART
left join SERI on STKA.V6PART = SERI.HTPART
left join OCRI on STKA.V6PART = OCRI.DDPART
Is this the best way to join the tables?

Answer 1

I think you already have your answer, but conceptually, there are a few issues here to deal with, and I figured I would give you a few examples, using data a little bit like yours, but massively simplified.
CREATE TABLE #STKA (V6PART INT, OTHER_DATA VARCHAR(50));
CREATE TABLE #METHDM (AQPART INT, KIT_ID INT, SOME_DATE DATETIME, OTHER_DATA VARCHAR(50));
CREATE TABLE #SERI (HTPART INT, OTHER_DATA VARCHAR(50));
CREATE TABLE #OCRI (DDPART INT, OTHER_DATA VARCHAR(50));
INSERT INTO #STKA SELECT 1, NULL UNION ALL SELECT 2, NULL UNION ALL SELECT 3, NULL; --1, 2, 3 Ids
INSERT INTO #METHDM SELECT 1, 1, '20200108 10:00', NULL UNION ALL SELECT 1, 2, '20200108 11:00', NULL UNION ALL SELECT 2, 1, '20200108 13:00', NULL; --1 Id appears twice, 2 Id once, no 3 Id
INSERT INTO #SERI SELECT 1, NULL UNION ALL SELECT 3, NULL; --1 and 3 Ids
INSERT INTO #OCRI SELECT 1, NULL UNION ALL SELECT 4, NULL; --1 and 4 Ids
So fundamentally we have a few issues here:
o the first problem is that the IDs in the tables differ, one table has an ID #4 but this isn't in any of the others;
o the second issue is that we have multiple rows for the same ID in one table;
o the third issue is that some tables are "missing" IDs that are in other tables, which you already covered by using LEFT JOINs, so I will ignore this.
--This will select ID 1 twice, 2 once, 3 once, and miss 4 completely
SELECT
*
FROM
#STKA
LEFT JOIN #METHDM ON #METHDM.AQPART = #STKA.V6PART
LEFT JOIN #SERI ON #SERI.HTPART = #STKA.V6PART
LEFT JOIN #OCRI ON #OCRI.DDPART = #STKA.V6PART;
So the problem here is that we don't have every ID in our "anchor" table STKA, and in fact there's no single table that has every ID in it. Now your data might be fine here, but if it isn't then you can simply add a step to find every ID, and use this as the anchor.
--This will select each ID, but still doubles up on ID 1
WITH Ids AS (
SELECT V6PART AS ID FROM #STKA
UNION
SELECT AQPART AS ID FROM #METHDM
UNION
SELECT HTPART AS ID FROM #SERI
UNION
SELECT DDPART AS ID FROM #OCRI)
SELECT
*
FROM
Ids I
LEFT JOIN #STKA ON #STKA.V6PART = I.Id
LEFT JOIN #METHDM ON #METHDM.AQPART = I.Id
LEFT JOIN #SERI ON #SERI.HTPART = I.Id
LEFT JOIN #OCRI ON #OCRI.DDPART = I.Id;
That's using a common-table expression, but a subquery would also do the job. However, this still leaves us with an issue where ID 1 appears twice in the list, because it has multiple rows in one of the sub-tables.
One way to fix this is to pick the row with the latest date, or any other ORDER you can apply to the data:
--Pick the best row for the table where it has multiple rows, now we get one row per ID
WITH Ids AS (
SELECT V6PART AS ID FROM #STKA
UNION
SELECT AQPART AS ID FROM #METHDM
UNION
SELECT HTPART AS ID FROM #SERI
UNION
SELECT DDPART AS ID FROM #OCRI),
BestMETHDM AS (
SELECT
*,
ROW_NUMBER() OVER (PARTITION BY AQPART ORDER BY SOME_DATE DESC) AS ORDER_ID
FROM
#METHDM)
SELECT
*
FROM
Ids I
LEFT JOIN #STKA ON #STKA.V6PART = I.Id
LEFT JOIN BestMETHDM ON BestMETHDM.AQPART = I.Id AND BestMETHDM.ORDER_ID = 1
LEFT JOIN #SERI ON #SERI.HTPART = I.Id
LEFT JOIN #OCRI ON #OCRI.DDPART = I.Id;
Of course you could also add some aggregation (SUM, MAX, MIN, AVG, etc.) to fix this problem (if it is indeed an issue). Also, I used a common-table expression, but this would work just as well with a subquery.

Answer 2

Expanding on a comment made on the question..
I would say I will start with SERI as that table contains the entire inventory for our facility and should cover the other tables
However the question said
SERI is our inventory table with 37180 records. (active inventory that is available)
In my experience, active inventory, isn't the same as all parts.
Normally, in a query like this, I'd expect the first table to be a Parts Master table of some sort that contains every possible part ID.