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'
Related
I want to understand when to open and close brackets when representing joins in MS Access queries as I am developing a query builder using C++ for MS Access queries so that I can apply the same code to generate similar queries.
SELECT
MasterTool.Name, Toolsets.SlaveToolID, Tools.MachineID
FROM
Tools AS MasterTool
LEFT JOIN
(
Toolsets LEFT JOIN Tools ON Toolsets.SlaveToolID = Tools.ID
)
ON MasterTool.ID = Toolsets.MasterToolID
Edit:
#LeeMac as per your explaination when i modified the query which i presented earlier to this
SELECT Tools.Name, Toolsets.SlaveToolID, Tools.MachineID FROM (Tools
LEFT JOIN Toolsets ON Toolsets.SlaveToolID = Tools.ID )
LEFT JOIN Tools ON Toolsets.MasterToolID = Tools.ID
i am getting error Join Expression Not Supported is there is any simple way to write the above query.
Essentially, when an MS Access query references more than two tables, every successive join between a pair of tables should be nested within parentheses.
For example, a query with two tables requires no parentheses:
select *
from a inner join b on a.id = b.id
The addition of a third joined table necessitates parentheses surrounding the original join in order to distinguish it from the additional join:
select *
from
(
a inner join b on a.id = b.id
)
inner join c on a.id = c.id
Every successive addition of a table will then cause the existing joins to be nested within another level of parentheses:
select *
from
(
(
a inner join b on a.id = b.id
)
inner join c on a.id = c.id
)
inner join d on a.id = d.id
Hence, in general:
select *
from
(
(
(
(
table1 [inner/left/right] join table2 on [conditions]
)
[inner/left/right] join table3 on [conditions]
)
[inner/left/right] join table4 on [conditions]
)
...
)
[inner/left/right] join tableN on [conditions]
There is a subtlety where LEFT/RIGHT joins are concerned, in that the order of nesting must maintain the direction of the join, for example:
select *
from
(
c left join b on c.id = b.id
)
left join a on a.id = b.id
Could be permuted to:
select *
from
c left join
(
b left join a on b.id = a.id
)
on c.id = b.id
I am using MS SQL SERVER 2008R2. i have two tables A and B as
create table A(
id int primary key, name Varchar(20));
create table B(
id int primary key, user_name Varchar(20));
insert into A values(1,'A1');
insert into A values(2,'A2');
insert into A values(3,'A3');
insert into A values(4,'A4');
insert into A values(5,'A5');
Now my problem is :
select A.*
from A left outer join B on A.id = B.id
where B.user_name like '%';
or
select A.*
from A left outer join B on A.id = B.id
where B.user_name like '%%';
Above written query does not return any records even though left table have 5 entries in it. without any filter on right table it works fine.
select A.* from A left outer join B on A.id = B.id
this query will give you out put like this...
id name id user_name
1 A1 NULL NULL
2 A2 NULL NULL
3 A3 NULL NULL
4 A4 NULL NULL
5 A5 NULL NULL
and you are comparing username using like with null
select A.* from A left outer join B on A.id = B.id where B.user_name like '%%';
hence it will not give you any output
you should try following query
select A.*,b.* from A left outer join B on A.id = B.id where (b.user_name like '%%' or b.user_name is null)
In your scenario...first left join is happening it is finding 5 entries and then on that record set sql sever is applying filter of user_name and as user_name for all rows is null..no records are getting displayed.
you can change your query to
select A.* from A left outer join B on A.id = B.id where ISNULL(B.user_name,'') like '%%';
You are using wild card for comparing null values as well,
Use this,
SELECT a.* FROM a LEFT OUTER JOIN b ON a.id = b.id WHERE b.user_name LIKE '%' OR b.user_name IS NULL;
Since all values in table B are NULL, any wildcard match on NULL values will return NULL.
So the condition where B.user_name like '%'; translates into where NULL like '%'; which evaluates to NULL as NULL cannot be compared with any value.
select A.* from A left outer join B on
A.id = B.id where COALESCE(B.user_name,'') like '%%';
select A.* from A left outer join B on
A.id = B.id where COALESCE(B.user_name,'') like '%';
supporting sql fiddle : http://sqlfiddle.com/#!6/1ca91/8
Note that the COALESCE is ANSI, and therefore supported in Oracle, SQL Server and PostGres and does shortcut evaluation. n
Edit: Based on new information that this same query should work in all SQL Server, PostGres and Oracle. I am changing the SQL query to
use COALESCE instead which is supported in all
Unless you use ISNULL() and check like this where
ISNULL(B.user_name,'') like '%';
select A.* from A left outer join B on
A.id = B.id where ISNULL(B.user_name,'') like '%%';
select A.* from A left outer join B on
A.id = B.id where ISNULL(B.user_name,'') like '%';
See this fiddle
http://sqlfiddle.com/#!6/1ca91/6
Please try this one:
select A.* from A
left outer join (SELECT * FROM B where user_name like '%') X on A.id = X.id;
Say I have three tables in SQL server 2008 R2
SELECT a.*, b.*, c.*
FROM
Table_A a
RIGHT JOIN Table_B b ON a.id = b.id
LEFT JOIN Table_C c ON b.id = c.id
or
SELECT a.*, b.*, c.*
FROM
Table_A a
RIGHT JOIN Table_B b ON a.id = b.id
JOIN Table_C c ON b.id = c.id
also, does it matter if I use b.id or a.id on joining c?
i.e. instead of JOIN Table_C c ON b.id = c.id, use JOIN Table_C c ON a.id = c.id
Thank you!
If it doesn't change the semantics of the query, the database server can reorder the joins to run in whichever way it thinks is more efficient.
Usually, if you want to force a certain order, you can use inline view subqueries, as in
SELECT a.*, x.*
FROM
Table_A a
RIGHT JOIN
(
SELECT *, b.id as id2 FROM Table_B b
LEFT JOIN Table_C c ON b.id = c.id
) x
ON a.id = x.id2
According to the definitions:
JOIN
: Return rows when there is at least one match in both tables
LEFT JOIN Return all rows from the left table, even if there are no matches in the right table
RIGHT JOIN Return all rows from the right table, even if there are no matches in the left table
The first option would include all raws from the 1st Join on Tables a and b even if there are no matching ones in table c, while the second statement would show only raws which match ones in table c.
regarding the second question i guess it would make a difference, since the 1st join includes all ids from table b, even though there are no matching ones in table a, so once you change your Join creterium to a.id you will get a different set of ids than b.id.
Yes, you do need a LEFT JOIN after a RIGHT JOIN
See
http://sqlfiddle.com/#!3/2c079/5/0
http://sqlfiddle.com/#!3/2c079/6/0
If you don't, the (inner) JOIN at the end will cancel out the effect of your RIGHT JOIN.
That wouldn't make any sense to have a RIGHT JOIN if you don't care. And if you care, you will have to add a LEFT JOIN after it.
Can anyone please explain me how can we use join on the basis of condition.
Lets say i am filtering data on the basis of a condition now my concern is if a particular BIT type parameters value is 1 then the data set include one more join else return same as earlier.
Here is three tables A,B,C
now i want to make a proc which has the #bool bit parameter
if #bool=0
then
select A.* from A
inner join B on B.id=A.id
and if #bool=1
then
select A.* from A
INNER JOIN B on B.id=A.id
inner join C on C.id=A.id
Thanks In Advance.
What you have will work (certainly in a SPROC in MS SQL Server anyway) with minor mods.
if #bool=0 then
select A.* from A
inner join B on B.id=A.id
else if #bool=1 then -- Or just else if #boll is limited to [0,1]
select A.* from A
INNER JOIN B on B.id=A.id
inner join C on C.id=A.id
However, the caveat is that SQL parameter sniffing will cache a plan for the first path it goes down, which won't necessarily be optimal for other paths through your code.
Also, if you do take this 'multiple alternative query' approach to your procs, it is generally a good idea to ensure that the column names and types returned are identitical in all cases (Your query is fine because it is A.*).
Edit
Assuming that you are using SQL Server, an alternative is to use dynamic sql:
DECLARE #sql NVARCHAR(MAX)
SET #sql = N'select A.* from A
inner join B on B.id=A.id'
IF #bool = 1
SET #sql = #sql + N' inner join C on C.id=A.id'
sp_executesql #sql
If you need to add filters etc, have a look at this post: Add WHERE clauses to SQL dynamically / programmatically
select A.* from A
inner join B on B.id = A.id
left outer join C on C.id = A.id and #bool = 1
where (#bool = 1 and C.id is not null) or #bool = 0
The #bool = 1 "activates" the left outer join, so to speak, and turns it, in effect, into an inner join by applying it in the WHERE clause, too. If #bool = 0 then the left outer join returns nothing from C and removes the WHERE restriction.
Try the following query
SELECT A.*
FROM A
INNER JOIN B on B.id=A.id
INNER JOIN C on C.id=A.id and #bool=1
You do it using a union:
SELECT A.*
FROM A
INNER JOIN B on B.id=A.id
WHERE bool = 0
UNION ALL
SELECT A.*
FROM A
INNER JOIN B on B.id=A.id
INNER JOIN C on C.id=A.id
WHERE bool = 1
I'm assuming that bool is stored in table A or B.
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