Lets say I have a query where one field can appear in either Table A or Table B but not both. So to retrieve it I use Coalesce.
Something like
Select
...
Coalesce(A.Number,B.Number) Number
...
From Table A
Left Join Table B on A.C= B.C
Now lets say I want to join another table to that Number field
should I just do
Join Table Z on Z.Z = Coalesce(A.Number,B.Number)
Or is it better to wrap my original table in a query and join on the definite result. So something like
Select * from (
Select
...
Coalesce(A.Number,B.Number) Number
...
From Table A
Left Join Table B on A.C= B.C
) T
left join Table Z on Z.Number= T.Number
Does this make a difference?
if i were joining another table to the result of the first query instead of a sub query i would place the first part in a CTE whenever possible, i believe the performance would be the same as a subquery but CTEs are more readable in my opinion.
with cte1 as
(
Select
...
Coalesce(A.Number,B.Number) Number
...
From Table A
Left Join Table B
on A.C= B.C
)
select *
from cte1 a
Join Table Z
on Z.Z = a.number
Related
Whats the difference between using a LEFT OUTER JOIN, rather than a sub-query that starts with a WHERE NOT EXISTS (...)?
No they are not the same thing, as they will not return the same rowset in the most simplistic use case.
The LEFT OUTER JOIN will return all rows from the left table, both where rows exist in the related table and where they does not. The WHERE NOT EXISTS() subquery will only return rows where the relationship is not met.
However, if you did a LEFT OUTER JOIN and looked for IS NULL on the foreign key column in the WHERE clause, you can make equivalent behavior to the WHERE NOT EXISTS.
For example this:
SELECT
t_main.*
FROM
t_main
LEFT OUTER JOIN t_related ON t_main.id = t_related.id
/* IS NULL in the WHERE clause */
WHERE t_related.id IS NULL
Is equivalent to this:
SELECT
t_main.*
FROM t_main
WHERE
NOT EXISTS (
SELECT t_related.id
FROM t_related
WHERE t_main.id = t_related.id
)
But this one is not equivalent:
It will return rows from t_main both having and not having related rows in t_related.
SELECT
t_main.*
FROM
t_main
LEFT OUTER JOIN t_related ON t_main.id = t_related.id
/* WHERE clause does not exclude NULL foreign keys */
Note This does not speak to how the queries are compiled and executed, which differs as well -- this only addresses a comparison of the rowsets they return.
As Michael already answered your question here is a quick sample to illustrate the difference:
Table A
Key Data
1 somedata1
2 somedata2
Table B
Key Data
1 data1
Left outer join:
SELECT *
FROM A
LEFT OUTER JOIN B
ON A.Key = B.Key
Result:
Key Data Key Data
1 somedata1 1
2 somedata2 null null
EXISTS use:
SELECT *
FROM A WHERE EXISTS ( SELECT B.Key FROM B WHERE A.Key = B.Key )
Not Exists In:
SELECT *
FROM A WHERE NOT EXISTS ( SELECT B.Key FROM B WHERE A.Key = B.Key )
Result:
Key Data
2 somedata2
Left outer join is more flexible than where not exists. You must use a left outer join if you want to return any of the columns from the child table. You can also use the left outer join to return records that match the parent table as well as all records in the parent table that have no match. Where not exists only lets you return the records with no match.
However in the case where they do return the equivalent rows and you do not need any of the columns in the right table, then where exists is likely to be the more performant choice (at least in SQL server, I don't know about other dbs).
I suspect the answer ultimately is, both are used (among other constructs) to perform the relational operation antijoin in SQL.
I suspect the OP wanted to know which construct is better when they are functionally the same (ie I want to see only rows where there is no match in the secondary table).
As such, WHERE NOT EXISTS will always be as quick or quicker, so is a good habit to get into.
I'm a newbie, just learning SQL and have this question: I have two tables with the same columns. Some registers are in the two tables but others only are in one of the tables. To illustrate, suppose table A = (1,2,3,4), table B=(3,4,5,6), numbers are registers. I need to select all registers in table B if they are not in table A, that is result=(5,6). What query should I use? Maybe a join. Thanks.
You can either use a NOT IN query like this:
SELECT col from A where col not in (select col from B)
or use an outer join:
select A.col
from A LEFT OUTER JOIN B on A.col=B.col
where B.col is NULL
The first is easier to understand, but the second is easier to use with more tables in the query.
Select register from TABLE_B b
Where not exists (Select register from TABLE_A a where a.register = b.register)
I assumed you have a column named register in TABLE_A and TABLE_B
I have a table A joined with a table B which give me a result set.
I want to join a table C to the previous ones in order to restrict the result set. But in case there is no result with this join, I would like to have the same result set than before (without taking care of C).
Can you think of way to do that in SQL ?
SELECT *
FROM TableA
INNER JOIN TableB
ON TableA.ID = TableB.TableAID
LEFT JOIN TableC
ON TableC.ID = TableB.TableCID
This will return all rows from Tables A & B but only the rows from TableC where the ON criteria match.
Otherwise conditional joins don't really apply in standard SQL. If you are using SQL Server you can perform some stored procedure logic to check the results from TableC and if there are none then only get data from Table A & B. But this approach with be provider specific
Not possible with regular SQL since it involves logic.
Your best bet is to make a small script, e.g. (in pseudo code)
select * into #tmp from x inner join y inner join z where blabla;
if (exists (select * from #tmp))
BEGIN
select * from #tmp
END
else
BEGIN
select * from x inner join y where blabla;
END
Edit:
But if I were you, I would just always join with C using a LEFT JOIN, so you can see if the result was in one or the other result set...
e.g.
select x.*, y.*, case when z.id is null then 0 else 1 end from x inner join y left join z on blabla where blabla;
But that of course assumes you are able to alter the code path that reads the result.
I see a problem in the LEFT/OUTER JOIN methods. If you do it you could get some results that are in A and B but not in C. If I understand well the porpouse is join AB with C, I mean the result when crossing with C must include the three restrictions. So the #Cine solution is the apropiate to this case.
I have two tables, one has primary key other has it as a foreign key.
I want to pull data from the primary table, only if the secondary table does not have an entry containing it's key. Sort of an opposite of a simple inner join, which returns only rows that join together by that key.
SELECT <select_list>
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
WHERE B.Key IS NULL
Full image of join
From aticle : http://www.codeproject.com/KB/database/Visual_SQL_Joins.aspx
SELECT
*
FROM
primarytable P
WHERE
NOT EXISTS (SELECT * FROM secondarytable S
WHERE
P.PKCol = S.FKCol)
Generally, (NOT) EXISTS is a better choice then (NOT) IN or (LEFT) JOIN
use a "not exists" left join:
SELECT p.*
FROM primary_table p LEFT JOIN second s ON p.ID = s.ID
WHERE s.ID IS NULL
Another solution is:
SELECT * FROM TABLE1 WHERE id NOT IN (SELECT id FROM TABLE2)
SELECT P.*
FROM primary_table P
LEFT JOIN secondary_table S on P.id = S.p_id
WHERE S.p_id IS NULL
If you want to select the columns from First Table "which are also present in Second table, then in this case you can also use EXCEPT. In this case, column names can be different as well but data type should be same.
Example:
select ID, FName
from FirstTable
EXCEPT
select ID, SName
from SecondTable
This was helpful to use in COGNOS because creating a SQL "Not in" statement in Cognos was allowed, but it took too long to run. I had manually coded table A to join to table B in in Cognos as A.key "not in" B.key, but the query was taking too long/not returning results after 5 minutes.
For anyone else that is looking for a "NOT IN" solution in Cognos, here is what I did. Create a Query that joins table A and B with a LEFT JOIN in Cognos by selecting link type: table A.Key has "0 to N" values in table B, then added a Filter (these correspond to Where Clauses) for: table B.Key is NULL.
Ran fast and like a charm.
I'm trying to do
SELECT * FROM a, b
However, it doesn't return anything if one of the tables is empty. How do I make it so it returns 'a' even if the other one is empty?
Using two tables in the from clause is functionally equivalent to a cross join:
select *
from A
cross join
B
This returns a row of A for every row in B. When B is empty, the result is empty too. You can fix that by using a left join. With a left join, you can return rows even if one of the tables is empty. For example:
select *
from A
left join
B
on 1=1
As the condition 1=1 is always true, this is just like a cross join except it also works for empty tables.
SELECT * FROM a LEFT JOIN b ON a.ID = b.ID
Will return everything from a even if b is empty.
You should do a left join.
Like this
SELECT *
FROM A
LEFT JOIN B ON A.ID = B.ID
Then you receive the rows in A and the respective row in B if exists.
SELECT a.*, b.* FROM a LEFT JOIN b ON a.id = b.id
in this example id is just example name for join key
The query mentioned above display join of both tables if a contain 2 record and b contain 7 records it displays 7*2 = 14 records. In your case one of the table is empty( with 0 records), it will not display any data. If still you want to display data and tables are not having any relationship, you need to check if count of both tables greater that 0. Otherwise display records from only one table which is not empty.