ORA-00933: SQL command not properly ended with INSERT INTO...SELECT statement - sql

Question

I fix my problem by chance, but I really want to know why it works :),
Here's the thing:
I get the ORA-00933: SQL command not properly ended Error when I execute the following SQL statement:
INSERT INTO BASP_DX.QLR#GT(BDCDYH, QSZT)
SELECT NVL(e.BDCDYH, ' '),b.LIFECYCLE AS QSZT
FROM DJ_DY a
LEFT JOIN DJ_XGDJGL d
ON d.ZSLBH = a.SLBH
LEFT JOIN DJ_DJB e
ON e.SLBH = d.FSLBH
AND e.SLBH = '0123456789'
LEFT JOIN DJ_QLRGL f
ON f.SLBH = e.SLBH
AND f.QLRLX = 'Person1'
LEFT JOIN DJ_QLRGL b
ON b.SLBH = a.SLBH
AND (b.QLRLX = 'Person2' OR (b.QLRLX = 'Person3' AND b.QLRID = f.QLRID))
WHERE a.SLBH = '12345'
AND e.SLBH IS NOT NULL
-- add the condition to ensure that
-- this statement and the second statement get the same result
AND b.QLRID IS NOT NULL
AND (a.LIFECYCLE = '0' OR a.LIFECYCLE IS NULL);
I remove all the unnecessary insert value, related table and condition from the original SQL statement, to focus on the problem part.
Then I google it, from this post I know the causes may be:
An INSERT statement with an ORDER BY clause or an INNER JOIN
A DELETE statement with an INNER JOIN or ORDER BY clause
An UPDATE statement with an INNER JOIN
Apparently, these are not my type. I didn't use the INNER JOIN and ORDER BY, all I use is LEFT JOIN statement, so I wonder it might be reason that I set too many conditions with the LEFT JOIN statement (such as LEFT JOIN DJ_QLRGL b), so I try move the conditions after WHERE clause, it looks like this:
INSERT INTO BASP_DX.QLR#GT(BDCDYH, QSZT)
SELECT NVL(e.BDCDYH, ' '),b.LIFECYCLE AS QSZT
FROM DJ_DY a
LEFT JOIN DJ_XGDJGL d
ON d.ZSLBH = a.SLBH
LEFT JOIN DJ_DJB e
ON e.SLBH = d.FSLBH
AND e.SLBH = '0123456789'
LEFT JOIN DJ_QLRGL f
ON f.SLBH = e.SLBH
AND f.QLRLX = 'Person1'
LEFT JOIN DJ_QLRGL b
ON b.SLBH = a.SLBH
-- this conditions move to WHERE clause
WHERE a.SLBH = '12345'
AND e.SLBH IS NOT NULL
-- here is the original LEFT JOIN condition
AND (b.QLRLX = 'Person2' OR (b.QLRLX = 'Person3' AND b.QLRID = f.QLRID))
AND (a.LIFECYCLE = '0' OR a.LIFECYCLE IS NULL);
Then it works!
But why?
I just want to know the reason for this situation.
Solution
the problem is triangular join, the LEFT JOIN conditions can't contain the condition concerning both the self join table, in this case, it's b.QLRID = f.QLRID, so when I remove the b.QLRID = f.QLRID condition, it works.

Answer 1

Firstly, the join of "DJ_QLRGL b" is not LEFT any more,
as WHERE condition excludes rows with no b counterpart found