SQL Why is my SELECT selecting duplicate rows? - sql

Question

This is a sub query:
SELECT t1.element_mark, t1.element_length, t1.element_width, t1.element_height, t1.product, t1.mass FROM IMP_ELEMENT as t1 WHERE TRIM(t1.project) = '99999';
This is the result:
This is my second sub query:
SELECT t2.element_id, t2.building, t2.floor_id, t2.deleted, t2.ELEMENT_MARK
FROM IMP_MODEL_GEOMETRY as t2
WHERE TRIM(t2.project) = '99999' AND TRIM(t2.building) = '1' AND TRIM(CAST(t2.floor_id AS VARCHAR(MAX))) = '1' AND t2.deleted = 0
ORDER BY t2.ELEMENT_MARK;
This is the result:
Now i'd like to combine them:
SELECT t1.element_mark, t1.element_length, t1.element_width, t1.element_height, t1.product, t1.mass, t2.element_id, t2.building, t2.floor_id, t2.deleted
FROM IMP_ELEMENT as t1
LEFT JOIN IMP_MODEL_GEOMETRY as t2 ON t1.element_mark = t2.element_mark
WHERE TRIM(t2.project) = '99999' AND TRIM(t2.building) = '1' AND TRIM(CAST(t2.floor_id AS VARCHAR(MAX))) = '1' AND t2.deleted = 0
ORDER BY t2.element_id;
And this is the result:
So what do i want?
I want only one result per "element_id".
I first want to SELECT"element_id" and then i want to find a match in the second table. To find a match i will use "element_mark". What is wrong in my query? why do i get multiple element_id?

Answer 1

As suggested by one of the comments, you probably have missed the WHERE clause in the first table. Try this query instead:
SELECT
t1.element_mark, t1.element_length, t1.element_width, t1.element_height, t1.product, t1.mass, t2.element_id, t2.building, t2.floor_id, t2.deleted
FROM
IMP_ELEMENT as t1
LEFT JOIN
IMP_MODEL_GEOMETRY as t2
ON
t1.element_mark = t2.element_mark
WHERE
TRIM(t2.project) = '99999' AND
TRIM(t2.building) = '1' AND
TRIM(CAST(t2.floor_id AS VARCHAR(MAX))) = '1' AND
t2.deleted = 0 AND
TRIM(t1.project) = '99999'
ORDER BY t2.element_id;

Answer 2

When joining two tables, in your case with a left join, if there are multiple matches on the join condition, multiple rows will be in the result set. You are joining based on element_mark- so for the first record of the first query- 3 rows will be returned in your result. What I would suggest is put the second table at the left part of the query or do a RIGHT JOIN. This way, because element_mark looks unique in the first table, you will get one record per element_id.

Answer 3

If you want to keep all elements and get matching information -- if any -- from the second table, then you want the LEFT JOIN. However, all conditions on the second table need to be in the ON clause:
SELECT t1.element_mark, t1.element_length, t1.element_width, t1.element_height, t1.product, t1.mass,
t2.element_id, t2.building, t2.floor_id, t2.deleted
FROM IMP_ELEMENT t1 LEFT JOIN
IMP_MODEL_GEOMETRY t2
ON t1.element_mark = t2.element_mark AND
TRIM(t2.project) = '99999' AND
TRIM(t2.building) = '1' AND
TRIM(CAST(t2.floor_id AS VARCHAR(MAX))) = '1' AND
t2.deleted = 0
ORDER BY t2.element_id;
There is no WHERE clause because all the filters are on the second table.