Suppose I issue a query like this:
SELECT a.x, b.y FROM a LEFT JOIN b ON b.id = a.id
I also want to know if a row from b is actually joined or there are just placeholder NULL values supplied by LEFTJOIN. I guess I can determine it comparing values of a.id and b.id in the result, but is there a way to do this in the query itself?
I.e. I'd want something like
SELECT a.x, b.y, b_is_actually_joined FROM a LEFT JOIN b ON b.id = a.id
where values in the column b_is_actually_joined are 1 or 0 (for example).
Just check for NULL b.id:
SELECT a.x, b.y, b.id IS NOT NULL AS b_is_actually_joined
FROM a
LEFT JOIN b ON b.id = a.id
For Oracle SQL you can use NVL2 function:
SELECT a.id, b.*, NVL2(b.id, 1, 0) AS b_is_actually_joined
FROM a
LEFT JOIN b ON b.id = a.id
SQL Fiddle
This should work in Ms Sql Server:
select CAST((coalesce(b.id, 0)) as bit) as b_is_actually_joined FROM a
LEFT JOIN b ON b.id = a.id
I am unaware of a standard SQL solution for this
Related
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You are using an outdated SQL Syntax.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Use proper join syntax and not the outdated ones: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
Here's a simplified example of what I'm trying to do. I have two tables, A and B.
A B
----- -----
id id
name a_id
value
I want to select only the rows from A where ALL the values of the rows from B match a where clause. Something like:
SELECT * from A INNER JOIN B on B.a_id = A.id WHERE B.value > 2
The problem with the above query is that if ANY row from B has a value > 2 I'll get the corresponding row from A, and I only want the row from A if
1.) ALL the rows in B for B.a_id = A.id match the WHERE, OR
2.) There are no rows in B that reference A
B is basically a table of filters.
SELECT *
FROM a
WHERE NOT EXISTS
(
SELECT NULL
FROM b
WHERE b.a_id = a.a_id
AND (b.value <= 2 OR b.value IS NULL)
)
This should solve your problem:
SELECT *
FROM a
WHERE NOT EXISTS (SELECT *
FROM b
WHERE b.a_id = a.id
AND b.value <= 2)
Here is the way in which this is obtained.
Suppose that we have available a universal quantifier (parallel to EXISTS, the existential quantifier), with a syntax like:
FORALL table WHERE condition1 : condition2
(to be read: FORALL the elements of table that satisfy the condition1, then condition2 is true)
So you could write your query in this way:
SELECT *
FROM a
WHERE FORALL b WHERE b.a_id = a.id : b.value > 2
(Note that forall is true even when no element in b exists with a value of a.id)
Then we can transform the universal quantifier in the existential one, with a double negation, as usual:
SELECT *
FROM a
WHERE NOT EXISTS b WHERE b.a_id = a.id : NOT (b.value > 2)
In plain SQL this can be written as:
SELECT *
FROM a
WHERE NOT EXISTS (SELECT *
FROM b
WHERE b.a_id = a.id
AND (b.value > 2) IS NOT TRUE)
This technique is very handy in case of universal quantification.
Answering this question (which it seems you actually meant to ask):
Return all rows from A, where all rows in B with B.a_id = A.id also pass the test B.value > 2.
Which is equivalent to:
Return all rows from A, where no row in B with B.a_id = A.id fails the test B.value > 2.
SELECT a.* -- "rows from A" (so don't include other columns)
FROM a
LEFT JOIN b ON b.a_id = a.id
AND (b.value > 2) IS NOT TRUE -- safe inversion of logic
WHERE b.a_id IS NULL;
When inverting a WHERE condition carefully consider NULL. IS NOT TRUE is the simple and safe way to perfectly invert a WHERE condition. The alternative would be (b.value <= 2 OR b.value IS NULL) which is longer but may be faster (easier to support with index).
Select rows which are not present in other table
Try this
SELECT * FROM A
LEFT JOIN B ON B.a_id = A.id
WHERE B.value > 2 OR B.a_id IS NULL
SELECT * FROM A LEFT JOIN B ON b.a_id = a.id
WHERE B.a_id IS NULL OR NOT EXIST (
SELECT 1
FROM b
WHERE b.value <= 2)
SELECT a.is, a.name, c.id as B_id, c.value from A
INNER JOIN (Select b.id, b.a_id, b.value from B WHERE B.value > 2) C
on C.a_id = A.id
Note it is a poor practice to use select *. You shoudl only specify fields you need. IN this case, I might possibly remove the b.Id refernces becasue they are probably not needed. If you have a join there is a 100% chance you are wasting resouces sending data you don't need becasue the join fields will be repeated. That is why I did nto include a_id in the final result set.
If you prefer not to use EXISTS, you can use an outer join.
SELECT A.*
FROM
A
LEFT JOIN B ON
B.a_id = A.id
AND B.value <= 2 -- note: condition reversed!!
WHERE B.id IS NULL
This works by searching for the existence of a failing record in B. If it finds one, then the join will match, and the final WHERE clause will exclude that record.
If i write a sql:
select *
from a,b
where a.id=b.id(+)
and b.val="test"
and i want all records from a where corresponding record in b does not exist or it exists with val="test", is this the correct query?
You're much better off using the ANSI syntax
SELECT *
FROM a
LEFT OUTER JOIN b ON( a.id = b.id and
b.val = 'test' )
You can do the same thing using Oracle's syntax as well but it gets a bit hinkey
SELECT *
FROM a,
b
WHERE a.id = b.id(+)
AND b.val(+) = 'test'
Note that in both cases, I'm ignoring the c table since you don't specify a join condition. And I'm assuming that you don't really want to join A to B and then generate a Cartesian product with C.
Move the condition into the JOIN clause and use the ANSI standard join pattern.
SELECT NameYourFields,...
FROM A
LEFT OUTER JOIN B
ON A.ID = B.ID
AND B.VAL = 'test'
INNER JOIN C
ON ...
A LEFT OUTER JOIN is one of the JOIN operations that allow you to specify a join clause. It preserves the unmatched rows from the first (left) table, joining them with a NULL row in the shape of the second (right) table.
So you can do as follows :
SELECT
FROM a LEFT OUTER JOIN b
ON a.id = b.id
--Note that you have used double quote "test" which is not used for varchar in SQL you should use single quote 'test'
AND b.val = 'test';
SELECT * FROM abc a, xyz b
WHERE a.id = b.id
AND b.val = 'test'
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