Suppose I have following tables: T1,T2 and T3.
How could I rephrase the following query using only left joins.
Select *
From T1
Right join T2 On T1.FK2=T2.PK
Right join T3 On T1.FK3=T3.PK
Following attempt is not correct:
Select *
From T2
Left join T1 On T1.FK2=T2.PK
Left join T3 On T1.FK3=T3.PK
T3 is On the wrong Side of the join. Is the following possible:
Select *
From T2
Left join T3 On T1.FK3=T3.PK
Left join T1 On T1.FK2=T2.PK
I can't Find a way to put both tables 2 and 3 On the left Side of 1 and use the correspondent fields to join all tables? The last query uses fields of table 1 before this table is mentioned in the query.
Or something like this?
Select *
From T2
Left join (
T3 left join T1
On T1.FK3=T3.PK)
On T1.FK2=T2.PK
Apparently brackets can help to order your joins. I wonder if this is really documented, i've found Nothing at first glance in the mysql docs.
Following query is correct and does not have any subqueries:
Select T1.Id Ida, t2.id idb, T3.id idc FROM T3
LEFT JOIN
(T2
LEFT JOIN T1 ON (T1.ID = T2.ID))
ON (T1.ID= T3.ID);
You need to use a subquery to first join t1 with t2 and then join the result with t3:
SELECT T.ID1 ID1, T.ID2 ID2, T3.ID ID3 FROM T3
LEFT JOIN
(SELECT T1.ID ID1, T2.ID ID2 FROM T2
LEFT JOIN T1 ON (T1.ID = T2.ID)) T
ON (T.ID1 = T3.ID);
SQL Fiddle
The first way is just to reverse the order that the tables are mentioned:
Select *
from t3 left outer join
t2
on T1.FK3 = T3.PK left outer join
t1
on T1.FK2 = T2.PK
But this won't work, because the first condition is on t1 and not t2. And t2 hasn't yet been defined.
When working with chains of tables in left or right outer joins, only the first (or last) tables are important, because they "drive" the query. "Drive" in the sense that they provide all the values even when there are no matches. So, the following should do what you want:
Select *
from t3 left outer join
t1
on T1.FK3 = T3.PK left outer join
t2
on T1.FK2 = T2.PK;
Related
I basically want to join the result of two INNER JOINs.
On this scheme I want to get the three arrows results combined.
I've tried INNER / LEFT combinations but it doesn't do the trick.
I think a nested request could be the solution but how ?
Thanks
The answer was actually simple : UNION
SELECT t1.*
FROM
(SELECT t1.*
FROM table1 t1 JOIN table2 t2 ON t2.id = i.client_id
UNION
SELECT t1.*
FROM t1 t1 JOIN table3 t3 ON t1.id = t3.client_id) as q1
;
I'd use logic to express the condition T1.id exists in T2 or T3 more directly, and certainly avoid use of DISTINCT or UNION.
Options could be to use EXISTS directly (As this is immure to the possibility of duplication cause by 1:many joins)...
SELECT
t1.*
FROM
table1 t1
WHERE
EXISTS (SELECT * FROM table2 t2 WHERE t2.t1_id = t1.id)
OR
EXISTS (SELECT * FROM table3 t3 WHERE t3.t1_id = t1.id)
Or to LEFT JOIN twice and then exclude unwanted rows. (This assumes that the joins are never 1:many, which would introduce duplication, and the unwanted need for a DISTINCT.)
SELECT
t1.*
FROM
table1 t1
LEFT JOIN
table2 t2
ON t1.id = t2.t1_id
LEFT JOIN
table3 t3
ON t1.id = t3.t1_id
WHERE
t2.t1_id IS NOT NULL
OR
t3.t1_id IS NOT NULL
SELECT t1.*, IFNULL(t2.profile_id, t3.profile_id) AS `profile_id`
FROM table1 AS t1
LEFT JOIN table2 AS t2
ON t1.id = t2.some_coulmn
LEFT JOIN table3 AS t3
ON t1.id = t3.some_coulmn
LEFT JOIN table4 AS t4
ON profile_id = t4.some_column
I'm trying to use an alias (profile_id) inside my join condition. It fails. Is there a way to do this?
Column aliases defined in th SELECT clause cannot be used in the join conditions. This is because the FROM clause is evaluated before the SELECT clause.
If I followed you correctly, you probably want:
SELECT t1.*, IFNULL(t2.profile_id, t3.profile_id) AS `profile_id`
FROM table1 AS t1
LEFT JOIN table2 AS t2
ON t1.id = t2.some_coulm
LEFT JOIN table3 AS t3
ON t1.id = t3.some_coulm
LEFT JOIN table4 AS t4
ON IFNULL(t2.profile_id, t3.profile_id) = t4.some_column
it's my first time here and my first question so sorry if i ask something stupid.
Well my problem is for MySQL.
Let me explain
I have 4 tables (T1,T2,T3,T4)
T1 (t1_id,external_id) here i keep the id's from T2,T3,T4 and then i have
T2 (t2_id,name,etc...)
T3 (t3_id,name,etc...)
T4 (t4_id,name,etc...)
the T1 has already some entries from these tables.
T2,T3,T4 have nothing common beetwen them but T1 has cause it keeps their ID's.
so the query is
**SELECT T1.*, T2.*, T3.*, T4.*
FROM T1
INNER JOIN T2 ON T2.t2_id=T1.external_id
INNER JOIN T3 ON T3.t2_id=T1.external_id
INNER JOIN T4 ON T4.t2_id=T1.external_id**
and it doesn't show nothing.I tried with LEFT JOIN but nothing.... but if i do this
**SELECT T1.*, T2.*
FROM T1
INNER JOIN T2 ON T2.t2_id=T1.external_id**
it works and shows data.
I hope you understand my problem.
Thanks!
Give this a try:
SELECT T1.*, T2.*, T3.*, T4.*
FROM T1
INNER JOIN T2 ON T2.t2_id=T1.external_id
INNER JOIN T3 ON T3.t3_id=T1.external_id
INNER JOIN T4 ON T4.t4_id=T1.external_id
Note that I have changed the id for T3 & T4, I assume that t2_id does not exist in them?
You should have three columns to hold each table id on T1.
T1 (T1_id, T2_id, T3_id, T4_id)
So when you do the inner join would be like this:
SELECT * FROM T1
INNER JOIN T2 ON T2.t2_id = T1.t2_id
INNER JOIN T3 ON T2.t3_id = T1.t3_id
INNER JOIN T4 ON T2.t4_id = T1.t4_id
Thank you all for your answers i used LEFT JOIN and now it's working. But the problem was that it was a capital letter on my database and both INNER & LEFT was not working.
Inner isn't correct to use cause there isn't always an entry from t2,t3,t4.
Thank you again for your time and your support!
If I have
SELECT * FROM Table1 t1
LEFT JOIN Table2 t2 ON t1.id = t2.id
WHERE t1.user='bob';
Does the WHERE clause run after the two tables are JOINED?
How do I make it so it runs prior to the JOIN?
The where clause will be executed before the join so that it doesn't join unnecessary records. So your code is fine the way it is.
Change the WHERE to another JOIN condition
LEFT JOIN Table2 t2 on t1.id = t2.id AND t1.user='bob'
In my experience in a left join you cannot exclude records in the 'left' (t1) table in the ON-statement since - by definition - all t1 records will be included. The where statement does work as it will be applied to the result of the join afterwards.
I do not exactly know what you want to achieve but most probably an inner join suits your needs as well and then you can add the t1.user='bob' condition to the ON-statement.
But if Mosty Mostacho is correct, the location (WHERE vs ON) of the condition is not relevant for speed of execution.
You should just add t1.user='bob' condition to ON clause before other condition and it will be evaluated first:
SELECT * FROM Table1 t1
LEFT JOIN Table2 t2
ON t1.user='bob' AND t1.id = t2.id;
What you may use is table expression after FROM like this:
SELECT *
FROM (SELECT
id
FROM Table1
WHERE user = 'bob') AS t1
LEFT JOIN Table2 t2
ON t1.id = t2.id
you can do
SELECT *
FROM Table1 t1
LEFT JOIN Table2 t2
ON t1.id=t2.id AND t1.user='bob';
RIGHT JOIN was the solution:
SELECT cars.manufacturer, cars.year FROM cars
RIGHT JOIN (SELECT m.manufacturer FROM cars AS m ORDER BY m.year DESC LIMIT 3) subq
ON cars.manufacturer=subq.manufacturer
Haven't put it through the full rigors yet, but seems to work.
I have two tables:
T1
1,a
2,b
T2
2,ggg
3,hhh
I want the join between them to give me all fields:
1,a,null,null
2,b,2,ggg
null,null,3,hhh
MySQL doesn't have FULL OUTER JOIN, but you can emulate it e.g.:
SELECT * FROM T1 LEFT OUTER JOIN T2 ON T1.id = T2.id
UNION ALL
SELECT * FROM T1 RIGHT OUTER JOIN T2 ON T1.id = T2.id
WHERE T1.id IS NULL;
In general:
FULL OUTER JOIN = LEFT OUTER JOIN ∪ (RIGHT OUTER JOIN ∖ INNER JOIN)
You need to cut one inner join (in here from right join, but IMHO doesn't matter which one you choose), because both returns same inner joins. In here you have:
T1::
T2::
LEFT OUTER JOIN::
RIGHT OUTER JOIN::
INNER JOIN::
FULL OUTER JOIN::
If the tables have a (combination of) columns that is unique, you can build a list of ids in a subquery. Then you can use two outer joins to simulate a full outer join:
select *
from (
select col1
from t1
union
select col1
from t2
) ids
left join
t1
on ids.col1 = t1.col1
left join
t2
on ids.col1 = t2.col1