I am unsure as to how to resolve an ambiguous column reference when using an alias.
Imagine two tables, a and b that both have a name column. If I join these two tables and alias the result, I do not know how to reference the name column for both tables. I've tried out a few variants, but none of them work:
Attempt 1
SELECT a.name, b.name
FROM (a INNER JOIN b ON a.id = b.id) AS x
This doesn't work as a and b are out of scope.
Attempt 2
SELECT x.a.name, x.b.name
FROM (a INNER JOIN b ON a.id = b.id) AS x
SQL syntax doesn't work like that.
Attempt 3
SELECT x.name, x.name
FROM (a INNER JOIN b ON a.id = b.id) AS x
That's just plain ambiguous!
I'm all out of ideas - any help would be much appreciated.
don't enclose it with parenthesis since (a INNER JOIN b ON a.id = b.id) is not a complete query.
SELECT a.name AS A_Name,
b.name AS B_Name
FROM a INNER JOIN b
ON a.id = b.id
or (assuming) if you have longer tables names and you want to make it short,
SELECT a.name AS A_Name,
b.name AS B_Name
FROM longTableNameA a
INNER JOIN longTableNameB b
ON a.id = b.id
Related
I have come across a Join Error in Access SQL, when using multiple "ON" criteria's. I am unable to perform an ON clause on 2 different tables, for example:
select *
from
(((A
left join B on a.id = b.id)
left join c on c.id = b.id)
left join D
on (d.id = b.id) and (d.id = a.id)
That final join statement causes an error because I link table D on table B first, and then link Table D on Table A. If I choose to instead link table D on Table B again, then it resolves. However, I need to join it this way due to the certain data I need to link Table D on from both tables.
How can I more efficiently structure my query to achieve my results?
you may try this select * from A left join B on a.id = b.id left join c on c.id = b.id left join D on d.id = b.id and d.id = a.id
It's somewhat difficult to tell what you're trying to do exactly, but most likely, this is what you want:
select *
from
(((A
left join B on a.id = b.id)
left join C on c.id = b.id)
left join D on d.id = a.id)
Since you're trying a LEFT JOIN, there is no reason to link multiple ids to eachother.
Can anyone help me figure out the correct WHERE clause for the following scenario:
select A.name
from tableA A, tableB B
where A.id = B.id
and
(
B.field = 5
OR
B.hasNoRowForJoinedID
)
I feel like I'm missing something really obvious here in how to accomplish this, but I can't for the life of me put my finger on it.
You are using an outdated SQL Syntax. To perform the LEFT OUTER JOIN based your your request above, you can do the following:
SELECT A.name
FROM A
LEFT OUTER JOIN B ON A.id = B.id
WHERE (B.field = 5 OR B.field IS NULL)
Use proper join syntax and not the outdated ones:
select A.name
from tableA A
left join tableB B on A.id = B.id and B.field = 5
There are three tables, A,B and C having common columns(name and number)
Table A have 10 records(say x) which can be only from table B(say, y) and table C(say, z) (like, x = y+z).
In table A, there are some records whose value is 0 (zero)
I need to compare those zero value based records using column = name, with other two tables.
And check the column "number" for the same "name" is also zero (0) in table B and table C?
I tried to write the below sample query to test on my small set of 3 tables data- but for some reasons I am not able to get all the 10 records as a result?
SELECT a.name,a.number as A_number, b.number as B_number, c.number as C_number
from A a, B b, C c
WHERE a.name = b.name
The above query gives me data as follows in the sqlfiddle-
http://sqlfiddle.com/#!2/57f86/1
In the above data- theres no record name="hello"
Can anyone please correct me where I am going wrong? and how to get the exact result? I need all the records from Table A. I know if I use left join it will populate all the left table data even if no match.
Possibilities: Table A having records, some may be present in table B
and some in table C, but not on both.
I think this is what you want:
SELECT a.*, b.number as bnumber, c.number as cnumber
from a left outer join
b
on a.name = b.name left outer join
c
on a.name = c.name
where a.number = 0;
By the way, here is a Postgres SQL Fiddle.
It's been over 20 years since the JOIN keyword was added to SQL. Use it:
select
a.name,
a.number as A_number,
b.number as B_number,
c.number as C_number
from A a
left join B b on a.name = b.name
left join C c on a.name = c.name
where a.number = 0
The key here is the use of left join, which allows all rows in table A to be returned, even if there are no matching rows in the other tables.
If you want to just display true/false if the number is zero in the other tables, do this:
select
a.name,
a.number as A_number,
(b.number = 0 and c.number = 0) as zero_elsewhere
from A a
left join B b on a.name = b.name
left join C c on a.name = c.name
where a.number = 0
When you wrote WHERE a.name = b.name, that restricted the records returned from table A to only those that also exist in table B. This is not equivalent to a left join. If you used only a WHERE statement you would need to do:
WHERE ((a.name = b.name) OR (b.name is NULL))
AND
((a.name = c.name) OR (c.name is NULL))
In the comments and other answers, they have been using LEFT JOIN which is easier to write and read. I suggest you adopt that style as it is widely accepted.
This is evidently correct syntax in SQL Server:
SELECT a.id, b.name
FROM Table1 a, Table2 b
WHERE a.id = b.fk1
So is this:
SELECT a.id, c.status
FROM Table1 a
JOIN Table3 c ON a.id = c.fk2
But this apparently isn't:
SELECT a.id, b.name, c.status
FROM Table1 a, Table2 b
JOIN Table3 c ON a.id = c.fk2
WHERE a.id = b.fk1
I would NOT normally want to construct a query in the third case's style (and really not the first case's either), but it would probably be the path of least resistence in editing some code that's already been written at my company. Somebody used the first form with five different tables, and I really need to work in a sixth table through a JOIN statement, without taking chances of messing up what they already have. Even though I could re-write their stuff outright if I need to, I would really like to know how to do something like in the third case.
Running the code exactly as-is in the examples, the third case gives me this error message:
The multi-part identifier "a.id" could not be bound.
What is syntactically breaking the third case? What simple fix could be applied? Thanks!
I, likewise, would not recommend doing this. But, you can just change the , to a cross join:
SELECT a.id, b.name, c.status
FROM Table1 a cross join Table2 b
JOIN Table3 c ON a.id = c.fk2
WHERE a.id = b.fk1
This code:
SELECT a.id, b.name, c.status
FROM Table1 a, Table2 b
JOIN Table3 c ON a.id = c.fk2
WHERE a.id = b.fk1
is doing a cross join on a and the result of an inner join on b and c. c cannot access any of the fields in a because the join is being performed on b. what you should do is change your query to:
SELECT a.id, b.name, c.status
FROM Table1 a
inner join Table2 b on a.id = b.fk1
inner JOIN Table3 c ON a.id = c.fk2
I need to simulate the left join effect without using the "left join" key.
I have two tables, A and B, both with id and name columns. I would like to select all the dbids on both tables, where the name in A equals the name in B.
I use this to make a synchronization, so at the beginning B is empty (so I will have couples with id from A with a value and id from B is null). Later I will have a mix of couples with value - value and value - null.
Normally it would be:
SELECT A.id, B.id
FROM A left join B
ON A.name = B.name
The problem is that I can't use the left join and wanted to know if/how it is possible to do the same thing.
you can use this approach, but you must be sure that the inner select only returns one row.
SELECT A.id,
(select B.id from B where A.name = B.name) as B_ID
FROM A
Just reverse the tables and use a right join instead.
SELECT A.id,
B.id
FROM B
RIGHT JOIN A
ON A.name = B.name
I'm not familiar with java/jpa. Using pure SQL, here's one approach:
SELECT A.id AS A_id, B.id AS B_id
FROM A INNER JOIN B
ON A.name = B.name
UNION
SELECT id AS A_id, NULL AS B_id
FROM A
WHERE name NOT IN ( SELECT name FROM B );
In SQL Server, for example, You can use the *= operator to make a left join:
select A.id, B.id
from A, B
where A.name *= B.name
Other databases might have a slightly different syntax, if such an operator exists at all.
This is the old syntax, used before the join keyword was introduced. You should of course use the join keyword instead if possible. The old syntax might not even work in newer versions of the database.
I can only think of two ways that haven't been given so far. My last three ideas have already been given (boohoo) but I put them here for posterity. I DID think of them without cheating. :-p
Calculate whether B has a match, then provide an extra UNIONed row for the B set to supply the NULL when there is no match.
SELECT A.Id, A.Something, B.Id, B.Whatever, B.SomethingElse
FROM
(
SELECT
A.*,
CASE
WHEN EXISTS (SELECT * FROM B WHERE A.Id = B.Id) THEN 1
ELSE 0
END Which
FROM A
) A
INNER JOIN (
SELECT 1 Which, B.* FROM B
UNION ALL SELECT 0, B* FROM B WHERE 1 = 0
) B ON A.Which = B.Which
AND (
A.Which = 0
OR (
A.Which = 1
AND A.Id = b.Id
)
)
A slightly different take on that same query:
SELECT A.Id, B.Id
FROM
(
SELECT
A.*,
CASE
WHEN EXISTS (SELECT * FROM B WHERE A.Id = B.Id) THEN A.Id
ELSE -1 // a value that does not exist in B
END PseudoId
FROM A
) A
INNER JOIN (
SELECT B.Id PseudoId, B.Id FROM B
UNION ALL SELECT -1, NULL
) B ON A.Which = B.Which
AND A.PseudoId = B.PseudoId
Only for SQL Server specifically. I know, it's really a left join, but it doesn't SAY LEFT in there!
SELECT A.Id, B.Id
FROM
A
OUTER APPLY (
SELECT *
FROM B
WHERE A.Id = B.Id
) B
Get the inner join then UNION the outer join:
SELECT A.Id, B.Id
FROM
A
INNER JOIN B ON A.name = B.name
UNION ALL
SELECT A.Id, NULL
FROM A
WHERE NOT EXISTS (
SELECT *
FROM B
WHERE A.Id = B.Id
)
Use RIGHT JOIN. That's not a LEFT JOIN!
SELECT A.Id, B.Id
FROM
B
RIGHT JOIN A ON B.name = A.name
Just select the B value in a subquery expression (let's hope there's only one B per A). Multiple columns from B can be their own expressions (YUCKO!):
SELECT A.Id, (SELECT TOP 1 B.Id FROM B WHERE A.Id = B.Id) Bid
FROM A
Anyone using Oracle may need some FROM DUAL clauses in any SELECTs that have no FROM.
You could use subqueries, something like:
select a.id
, nvl((select b.id from b where b.name = a.name), "") as bId
from a
you can use oracle + operator for left join :-
SELECT A.id, B.id
FROM A , B
ON A.name = B.name (+)
Find link :-
Oracle "(+)" Operator
SELECT A.id, B.id
FROM A full outer join B
ON A.name = B.name
where A.name is not null
I'm not sure if you just can't use a LEFT JOIN or if you're restricted from using any JOINS at all. But as far as I understand your requirements, an INNER JOIN should work:
SELECT A.id, B.id
FROM A
INNER JOIN B ON A.name = B.name
Simulating left join using pure simple sql:
SELECT A.name
FROM A
where (select count(B.name) from B where A.id = B.id)<1;
In left join there are no lines in B referring A so 0 names in B will refer to the lines in A that dont have a match
+ or A.id = B.id in where clause to simulate the inner join