MYSQL 两个内连接不产生结果

Question

I have 3 tables connected with id's constructed like this:

RAD table
rad_id      strp_ID     strf_ID
1               1           null
2               1           null
3           null            3
4           null            4
5           null            4

STRANKEP table
strp_ID     strp_NAZIV
1           data1
2           data2
3           data3

STRANKEF table
strf_ID     strf_NAZIV
1           data1
2           data2
3           data3
4           data4

I'm trying to get for example value of strf_NAZIV witch is data4 in case rad_id=4. Because rad_id=4 has strf_ID=4 and in STRANKEF table stf_ID=4 has data4 value.

Example quarry for rad_id=4 is:

SELECT rad_id, strp_NAZIV, strf_NAZIV FROM RAD 
INNER JOIN STRANKEP ON RAD.strp_ID=STRANKEP.strp_ID 
INNER JOIN STRANKEF ON RAD.strf_ID=STRANKEF.strf_ID 
WHERE rad_id = 4;

When I run the quarry I get 0 rows result with no errors and correct columns. I can not get my head around this, please advise.

rad_id strp_NAZIV strf_NAZIV
0 rows

Answer 1

Okay, the first thing is that inner join only returns results where an attribute of a tuple exists in both tables, when one of the table contains null values for eg RAD, the tuples with ID 3, 4, 5 won't be returned, from the result of first inner join and 1 and 2 from 2nd. For more reference:

https://www.w3schools.com/SQL/sql_join_inner.asp

Try using outer joins instead of inner to get the results with null values.

Answer 2

There is no STRANKEP table where strp_ID = 4 . When doing an INNER JOIN , it only keeps rows where both tables match.

Maybe you want this:

SELECT rad_id, strp_NAZIV, strf_NAZIV 
FROM RAD 
LEFT JOIN STRANKEP ON RAD.strp_ID=STRANKEP.strp_ID 
LEFT JOIN STRANKEF ON RAD.strf_ID=STRANKEF.strf_ID 
WHERE rad_id = 4;