SQLite: CASE within FROM section - sqlite

Question

I need to select the MCUs table if UserID = 7 does not exist in the table. Please help me, what am I doing wrong?
SELECT
MCUs.MCUID
FROM
CASE
WHEN ReqMCUDevs.UserID = 7 THEN
MCUs
INNER JOIN MCUDevs
ON MCUs.MCUID = MCUDevs.MCUID
INNER JOIN ReqMCUDevs
ON MCUDevs.DevID = ReqMCUDevs.DevID
AND ReqMCUDevs.Quantity >= MCUDevs.Quantity
ELSE
MCUs
END
I need the resunt ot this query if UserID=7 exists
SELECT
MCUs.MCUID
FROM
MCUs
INNER JOIN MCUDevs
ON MCUs.MCUID = MCUDevs.MCUID
INNER JOIN ReqMCUDevs
ON MCUDevs.DevID = ReqMCUDevs.DevID
AND ReqMCUDevs.Quantity >= MCUDevs.Quantity
WHERE
ReqMCUDevs.UserID = 7
and this query if not
SELECT
MCUs.MCUID
FROM
MCUs
database schema:
The easiest solution is to use two different queries
db = sqlite3.connect("..\\DB.db", isolation_level = None);
c = db.cursor()
#...
c.execute("SELECT UserID FROM ReqMCUDevs WHEN UserID = ?", (UserID,))
if c.fetchall() == []:
c.execute("SELECT MCUID FROM MCUs")
else:
c.execute("""SELECT MCUID FROM MCUDevs
JOIN ReqMCUDevs ON MCUDevs.DevID = ReqMCUDevs.DevID
AND ReqMCUDevs.Quantity <= MCUDevs.Quantity
WHERE ReqMCUDevs.UserID = ?""", (UserID,))
but it is really not the answer

Answer 1

CASE works only in expressions (i.e., to compute a value); it is not possible to change the basic structure of a query conditionally.
However, there are different mechanisms to get what you want.
First, the query with the joins does not actually return anything from the joined tables, so it is possible to rewrite it with a subquery:
SELECT Name,
Price
FROM MCUs
WHERE MCUID IN (SELECT MCUID
FROM MCUDevs
JOIN ReqMCUDevs ON MCUDevs.DevID = ReqMCUDevs.DevID
AND ReqMCUDevs.Quantity >= MCUDevs.Quantity
WHERE ReqMCUDevs.UserID = 7)
ORDER BY Price
Now, we want to ignore this WHERE filter if the subquery is empty.
This can be done with a separate check:
SELECT Name,
Price
FROM MCUs
WHERE MCUID IN (SELECT MCUID
FROM MCUDevs
JOIN ReqMCUDevs ON MCUDevs.DevID = ReqMCUDevs.DevID
AND ReqMCUDevs.Quantity >= MCUDevs.Quantity
WHERE ReqMCUDevs.UserID = 7)
OR NOT EXISTS (SELECT MCUID
FROM MCUDevs
JOIN ReqMCUDevs ON MCUDevs.DevID = ReqMCUDevs.DevID
AND ReqMCUDevs.Quantity >= MCUDevs.Quantity
WHERE ReqMCUDevs.UserID = 7)
ORDER BY Price