My goal is to populate several objects of several child classes (B, C, D, E, F...), all extending a parent class which is A. All of this, in one single and only big query.
Let's say I have several tables reflecting the classes structure, including these 3 ones:
Table A /* the parent class */
id type created
--------------------------------
392 B 1377084886
Table B /* one of the child classes */
id myfield myotherfield anotherone oneagain
-------------------------------------------------------------
392 234 'foo' 'bar' 3
Table G /* not part of the structure, just here for the query to check a field */
myfield fieldtocheck evenmorefields
------------------------------------------------
234 'myvalue1' 'foobar'
Now:
/* This (the query I want): */
SELECT
a.*,
b.*,
c.*,
d.*,
e.*,
f.*
FROM A a
LEFT JOIN B b ON a.id = b.id
LEFT JOIN C c ON a.id = c.id
LEFT JOIN D d ON a.id = d.id
LEFT JOIN E e ON a.id = e.id
LEFT JOIN F f ON a.id = f.id
LEFT JOIN G g_b ON b.myfield = g_b.myfield
LEFT JOIN G g_c ON c.myfield = g_c.myfield
WHERE g_b.fieldtocheck IN (myvalue1);
/* Returns this (what I don't want): */
id->392
type->B
created->1377084886
myfield->NULL /* why NULL? */
myotherfield->NULL /* why NULL? */
anotherone->NULL /* why NULL? */
oneagain->3 /* why, whereas other fields from B are NULL, is this one correctly filled? */
Whereas:
/* This (the query I don't want): */
SELECT
a.*,
b.*
FROM A a
LEFT JOIN B b ON a.id = b.id
LEFT JOIN G g_b ON b.myfield = g_b.myfield
WHERE g_b.fieldtocheck IN (myvalue1);
/* Returns this (what I want): */
id->392
type->B
created->1377084886
myfield->234
myotherfield->'foo'
anotherone->'bar'
oneagain->3
I have no idea why. Tried different things, but this is what I come up with. Has someone an idea?
EDIT: Clarified this post and made it more straightforward.
I think the problem that you are facing is the collision of names in the query. That is, there are multiple columns with the same name and MySQL chooses one of them for the result.
You need to alias the column names for each of the tables to a different name, for example, a_created, b_created, etc.
This appears to be the classic case where you need to move your where into the join itself so that it doesn't block everything else:
SELECT
A.*,
B.*,
C.*,
D.*,
E.*,
F.*,
FROM A
LEFT JOIN B ON A.id = B.id
LEFT JOIN C ON A.id = C.id
LEFT JOIN D ON A.id = D.id
LEFT JOIN E ON A.id = E.id
LEFT JOIN F ON A.id = F.id
LEFT JOIN G ON B.myfield = G.myfield and G.anotherfield IN (myvalue1)
LEFT JOIN H ON C.myfield = H.myfield;
SELECT a.id, a.type, a.created, b.myfield, b.myotherfield FROM `A` AS a INNER JOIN `B` AS b ON a.id = b.id ;
Use this It must work fine. If you have an additional where query you can add it.
Related
There are 3 tables. Lets call them a, b and c. Selected data from table a must be intersected with b or c depending of value of one cell in table a. Lets call this cell d. Is there way in MySQL to make query like:
SELECT
a.cell,
a.other_cell,
a.d,
alias.cell,
alias.other_cell if(
a.d = 3,
left join b as alias on b.id = a.id,
left join c as alias on c.id = a.id
)
FROM
a where a.id = 123
You can try for example Conditional join as here but instead coalesce, use CASE WHEN
In short:
Instead, you simply LEFT OUTER JOIN to both tables, and in your SELECT clause, return data from the one that matches
select
E.EmployeeName,
CASE
WHEN d = 3 THEN s.store
WHEN d <>3 THEN o.office
END as Location
from
Employees E
left outer join
Stores S on …
left outer join
Offices O on …
NOTE I think the two columns must have the same type, at least in PG, not sure about MYSQL
Assuming you want select from the different tables (b and c) based on a 'cell' value in
table a then something like this work may work for you.
SELECT
a.cell,
a.other_cell,
a.d,
if(a.d = 3, b.cell, c.cell) cell
FROM a
INNER JOIN b on b.id = a.id
INNER JOIN c on c.id = a.id
WHERE a.id = 123
This should join the all 3 tables provided the id in b and c also match the id in table a. The if function will select the cell column from table b or c based on the value of column d in table a.
Also helpful: https://www.w3resource.com/mysql/control-flow-functions/if-function.php
I need to do something like this (in MySQL), my attempts using UNION won't work until now.
In theory:
SELECT * FROM
tableA A
JOIN tableB B ON A.tableAId = B.tableAId
LEFT JOIN tableC C ON C.tableAId = A.tableAId
LEFT JOIN tableD D ON D.tableAId = A.tableAId
JOIN tableE E
ON (C.tableEId = E.tableEId OR D.tableEId = E.tableEId)
The expected result is crossing A with B, optional C with A, optional D with A and get the E result if its id is on C or D.
In order to know if the record found is from C or D, I'm using an IF checking if tableCId or tableDId are null.
I think your code will work.
SELECT *
FROM tableA A JOIN
tableB B
ON A.tableAId = B.tableAId LEFT JOIN
tableC C
ON C.tableAId = A.tableAId LEFT JOIN
tableD D
ON D.tableAId = A.tableAId JOIN
tableE E
ON E.tableEId IN (C.tableEId, D.tableEId);
This will filter out rows that have no matches in C and D, as well as those whose matches in C/D are not in E. I assume that is desirable.
Also, OR and IN can have a bad impact on JOIN performance, but you don't mention performance as a concern.
Let's say that I have:
SELECT * FROM a
LEFT OUTER JOIN b ON b.a_id = a.id
LEFT OUTER JOIN c ON b.c_id = c.id
Now what I want to do is to select b's that are assigned to c that is e. g. active (c.active = 1). How can I do that with ON?
Note that I can't use WHERE after the whole query above, because I want a's to be returned even if 0 b's are found.
Just to make sure, I understood the question: You want all rows where either c.active equals 1 or where there is no entry in b or c, right?
It's a bit lengthy but this seems to work:
SELECT * FROM a
LEFT OUTER JOIN b ON a.aid = b.aid
LEFT OUTER JOIN c ON b.bid = c.bid
WHERE a.aid NOT IN (
SELECT a.aid FROM a
INNER JOIN b ON a.aid = b.aid
INNER JOIN c ON b.bid = c.bid
WHERE NOT c.active
);
I could also imagine a solution using UNION
Which rows are returned in each case... Note that one case is different:
FROM b JOIN c ON ... AND c.active=1 -- rows in both tables exist and active
FROM b JOIN c ON ... WHERE c.active=1 -- ditto
FROM b LEFT JOIN c ON ... WHERE c.active=1 -- ditto
FROM b LEFT JOIN c ON ... AND c.active=1 -- all b's, but NULLs for inactive/missing c's
(Caveat: I am not sure I got the cases correct; just keep in mind that ON and WHERE are not always interchangeable.)
When mixing JOIN and LEFT JOIN, you may need to add parentheses:
FROM a JOIN ( b LEFT JOIN c ON... ) ON ... WHERE ...
FROM ( a JOIN b ON ... ) LEFT JOIN c ON... WHERE ...
Can anyone help me with this query?
I have three tables (A;B;C)
A <--1....N---> B <--1....N---> C
I want all A rows having C.dates (the greatest)
SELECT A.*, MAX(C.dates)
FROM A
JOIN B ON B.A_fk = A.id
JOIN C ON C.B_fk = B.id
GROUP BY A.id
This JOIN will exclude results which wont have LEFT join. That is, if any row from A wont have B row or any row from B wont have any C row, then the row wont show. To overcome this you can use LEFT JOIN instead of JOIN.
SELECT A.*, MAX(C.dates)
FROM A
LEFT JOIN B ON B.A_fk = A.id
LEFT JOIN C ON C.B_fk = B.id
GROUP BY A.id
EDIT: Sorry didnt noticed that you needed the greatest value of C.data. There you have it. You have to use MAX function in SELECT and GROUP BY A.id
I'm coming across this situation alot, I'll have a query that will have one table needed in a join condition that may have no entries therefore requiring me to use a LEFT JOIN. I can't wrap my head around the syntax when it's used with more than 1 join.
I'll have:
SELECT A.*, B.*, C.*
FROM A, B, C
WHERE A.id = C.id
AND C.aid = A.id
AND B.cid = C.id
Along comes D with the possibility of being empty and I have to rewrite the query and run into problems.
How can I simply join D to any one of these tables?
You're much better off explicitly specifying all of your JOINs. That should make things much clearer.
SELECT A.*, B.*, C.*, D.*
FROM A
INNER JOIN C
ON C.aid = A.id
INNER JOIN B
ON B.cid = C.id
LEFT JOIN D
ON C.did = d.id
My advice is to never specify more than one column on FROM clause.
For clarity, it's better to always:
Use JOIN clause
Use aliases
Specify columns of joined tables on left side of equal sign
Example:
SELECT a.*, b.*, c.*
FROM ATable a
INNER JOIN BTable b
ON b.id = a.id
INNER JOIN CTable c
ON c.id = a.id
WHERE a.someColumn = 'something'
Not sure about MySQL, but in some other SQL flavors, you can use the same on UPDATES and DELETES, like:
DELETE FROM a
FROM ATable a
INNER JOIN BTable b
ON b.id = a.id
INNER JOIN CTable c
ON c.id = a.id
WHERE a.someColumn = 'something'
or
UPDATE a
SET something = newValue
FROM ATable a
INNER JOIN BTable b
ON b.id = a.id
INNER JOIN CTable c
ON c.id = a.id
WHERE a.someColumn = 'something'
The syntax below should help you. The basic premise is whatever table is listed LEFT is the required.. the table (or alias) on the right is optional. I understand you don't quite get it, and your syntax sample shows that (not meant to criticize) as you are joining from A -> C and C back to A on a different field. If this is the case where two fields are in the "C" table that BOTH point to A, you would re-join to A as a second alias...
select
Want.*,
Maybe.*,
SecondA.*,
B.*
From
A as Want
LEFT JOIN C as Maybe
on Want.ID = Maybe.ID
JOIN A as SecondA
on Maybe.AID = SecondA.ID
JOIN B
on Maybe.ID = B.cID
So, this query is stating I want everything from Table A (alias Want -- left side/first table in the list) Regardless of there being a match in Table C (alias Maybe) where the ID keys match.
Notice the next joins going down from "C" back to the second instance of "A" and table B. I have those as just joins... So the relationship between the "Maybe" alias, and that of second instance of "A" and "B" are JOIN (required).
Hopefully this gives some better clarification on HOW it works.
Now, for your real-life query. If you can describe what you are looking for, and your sample table structures / result expections, listing that could offer more explicit solution to your needs.
Hope this will help
SELECT
A.*, B.*, C.*
FROM A
inner join C on(A.id = C.id)
inner join B on(B.cid = C.id)