Why oracle doesn't recognize this sentence ? It says "From keyword" wasn't found where expected. What's wrong with it ?
Example:
select distinct a.id = b.id
from table1 a, table2 b
where a.column = X and b.column =Y;
MySQL allows me to do that. So what should I change ?
Your problem is that a.id = b.id is not valid sql when it's in the select clause.
Edit Below
Given your comment about expecting a boolean result, maybe you are looking for a case construct.
select case when a.id = b.id then 1 else 0 end BooleanResult
from tablea a join tableb b on something
where etc
First off, Oracle does not have a boolean data type in SQL (there is a boolean data type in PL/SQL) so a query cannot return a boolean.
You can do something like
select distinct (case when a.id = b.id
then 1
else 0
end)
from table1 a,
table2 b
where a.column = X
and b.column = Y;
It strikes me as terribly unlikely, however, that you really want to do a Cartesian product between table1 and table2 only to then apply a DISTINCT operator. Frequently, people incorrectly add a DISTINCT to a query when what they really want to do is add another join condition. I would expect that you really want
select distinct (case when a.id = b.id
then 1
else 0
end)
from table1 a,
table2 b
where a.some_key = b.some_key
and a.column = X
and b.column = Y;
Once you have the join defined correctly, you may no longer need the expense of the extra DISTINCT.
Here is it
select distinct a.id, b.id
from table1 a, table2 b
where a.column = X and b.column =Y;
You can also use decode in oracle
select distinct (decode (a.id,b.id,1,0)) from table1 a, table2 b
where a.some_key = b.some_key
and a.column = X ;
Related
In my SELECT statement, I join two tables. It's possible that there's no corresponding value in Table2 in which case, my SELECT statement should return NULL for that column. What I want to do is that instead of returning the actual value coming from Table2 I want to return a 1.
I thought I could use ISNULL for this but that function is designed to return the actual value if one exists.
This is what my SELECT statement looks like:
SELECT a.Id, ISNULL(b.PersonId, 0)
FROM Table1 AS a
LEFT OUTER JOIN Table2 AS b ON a.Id = b.Id
Here the PersonId column is of UNIQUEIDENTIFIER type and I don't want to return either a Guid or a 0. Instead, I'd like to return a 1 or a 0.
How can I handle this scenario?
I think you're looking for a CASE EXPRESSION here.
SELECT a.Id, CASE WHEN b.PersonId IS NOT NULL THEN 1 ELSE 0 END
FROM Table1 AS a
LEFT OUTER JOIN Table2 AS b ON a.Id = b.Id
As well as #DaleK's excellent answer, another option is using EXISTS
SELECT
a.Id,
CASE WHEN EXISTS (SELECT 1
FROM Table2 AS b
WHERE a.Id = b.Id)
THEN 1 ELSE 0 END
FROM Table1 AS a
Note that the semantic is different here, as it is a semi-join: the rows are not duplicated if there are multiple matches in Table2
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.
Hopefully someone can show me how to complete my sql. I'll start by showing you my existing query:
Select a.ID, a.NAME,
CASE When b.SOURCE = Value1 then 'Rename1'
When b.SOURCE = Value2 then 'Rename2'
When b.SOURCE = etc. ....
END,
b.TARGET, b.COMPARE
FROM Table'a', Table'b'
WHERE b.SOURCE in(Value1, Value2, Value...etc)
AND (a.ID = b.ID_1 or a.ID = b.ID_2)
Now my issue is my client wants to look up 3 different tables (each table contains primary key = a.ID) and return the values from only the column values that are referenced in the Case Statement above.
I assume I would need to do a join but not sure how to go about it. Any suggestions?
Thanks!
Puckhog
Following is the sample query:
Select T1.column,T2.Column,T3.Column
From Table1 T1
Join Table2 T2
on T1.MatchingColumn=T2.MatchingColumn
Join Table3 T3
on T2.MatchingColumn=T3.MatchingColumn
You could do something like this?
SELECT
a.ID, a.NAME, c.SOMETHING
FROM
a
JOIN
b
ON a.ID = b.ID
JOIN
c
ON a.ID = c.ID
WHERE [conditions]
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
I'm trying construct a PostgreSQL query that does the following but so far my efforts have been in vain.
Problem:
There are two tables: A and B. I'd like to select all columns from table A (having columns: id, name, description) and substitute the "A.name" column with the value of the column "B.title" from table B (having columns: id, table_A_id title, langcode) where B.table_A_id is 5 and B.langcode is "nl" (if there are any rows).
My attempts:
SELECT A.name,
case when exists(select title from B where table_A_id = 5 and langcode= 'nl')
then B.title
else A.name
END
FROM A, B
WHERE A.id = 5 and B.table_A_id = 5 and B.langcode = 'nl'
-- second try:
SELECT COALESCE(B.title, A.name) as name
from A, B
where A.id = 5 and B.table_A_id = 5 and exists(select title from B where table_A_id = 5 and langcode= 'nl')
I've tried using a CASE and COALESCE() but failed due to my inexperience with both concepts.
Thanks in advance.
araqnid's is the answer you are looking for, I bet.
But if you want to enforce that no more than one row is returned for each original matching A row, you might prefer to do a subselect instead of a LEFT JOIN. For example:
SELECT A.id, COALESCE(
( SELECT max(B.title) FROM B WHERE
langcode = 'nl' AND B.table_a_id = A.id), A.name ) as name
FROM A
WHERE A.id = 5
I use "max" here to select an arbitrary value, in the event there is more than one. You can use "min" or whatever you consider appropiate in your case.
Perhaps this is more easy to understand than the LEFT JOIN, but (apart from the two being not exactly equivalent) a JOIN will perform better than N subselects (much better is N is large).
Anyway, from a learning point of view, it's good to understand both.
select A.id, coalesce(B.title, A.name)
from TableA AS A
left join (select table_a_id, title from TableB where langcode = 'nl') AS B
on B.table_a_id = A.id
WHERE A.id = 5
Ok, I'm not sure how your tables have to be joined, but something like this should do the job:
SELECT yourcolumnlist,
CASE WHEN A.name IS NULL THEN B.title ELSE A.name END
FROM TableA AS A
INNER JOIN TableB AS B
ON A.id = B.table_A_id
WHERE B.table_A_id = 5
AND B.langcode = 'nl'
Another way to do it would be to use the COALESCE() function:
SELECT yourcolumnlist,
COALESCE(A.name, B.title)
FROM TableA AS A
INNER JOIN TableB AS B
ON A.id = B.table_A_id
WHERE B.table_A_id = 5
AND B.langcode = 'nl'
Try this
select A.id,B.title,A.description from TableA as A inner join tableB as B
on A.id=B.id where B.table_A_id = 5 and B.langcode ='nl'