I have a table-valued function with joins where I want to choose which join I use depending on a local variable like:
DECLARE #type int;
Then do some logic with #type and set it to 1.
SELECT ...
FROM table t
inner join ... a on a.id = t.id and #type = 1 -- Only trigger this join if #type is 1
inner join ... b on b.id = t.id and #type = 2 -- Only trigger this join if #type is 2
So my question is: how can I choose which join to trigger depending on the value of #type (if even possible).
The reason I want to do this is that the SELECT statement is massive, and I don't want repetitive code in the script.
Use left join instead:
SELECT ...
FROM table t LEFT JOIN
a
ON a.id = t.id AND #type = 1 LEFT JOIN
b
ON b.id = t.id AND #type = 2 ;
You might need WHERE #type IN (1, 2) if you want an empty result set for other values.
You will need COALESCE() in the SELECT to combine the columns:
COALESCE(a.col1, b.col1) as col1
This should be quite efficient. However, you might want to simply use UNION ALL:
SELECT ...
FROM table t JOIN
a
ON a.id = t.id
WHERE #type = 1
UNION ALL
SELECT ...
FROM table t JOIN
b
ON b.id = t.id
WHERE #type = 2 ;
You could union your two tables within a subquery. For any similar columns (i.e. would be in the same column in the outer select) you can place them above each other, for columns unique to each source you'd need to pad the other side of the union with NULL, e.g.
SELECT t.id,
a.SimilarCol,
a.UniqueToA,
a.UniqueToB
FROM Table AS t
INNER JOIN
( SELECT a.id,
a.SimilarCol, -- Column you would want to consider the same in each table
a.UniqueToA, -- Column Unique to this table
UniqueToB = NULL -- Column Unique to the other table
FROM SomeTable AS a
WHERE #Type = 1
UNION ALL
SELECT b.id,
b.SimilarCol,
UniqueToA = NULL,
b.UniqueToB
FROM SomeOtherTable AS b
WHERE #type = 2
) AS a
ON a.id = t.id;
Example on db<>Fiddle
Related
Does SQL allow some form of conditional "table choosing" within the Join statement? ie. selecting a different table to join based on a predefined variable/condition.
define var = 1
select *
from tbl
join (case when &var=1 then tblA when &var=2 then tblB else tblC end) a on tbl.id = a.id
The error I get when attempting this method is ORA-00905: missing keyword.
No. Neither SQL nor Oracle allow this, unless you use dynamic SQL.
Assuming the tables have the same columns, you could write this logic as:
select *
from tbl join
(select a.* from tblA where &var = 1 union all
select b.* from tblB where &var = 2 union all
select c.* from tblC where &var not in (1, 2)
) abc
on tbl.id = abc.id;
You still need to specify all joins ahead of time and they would have to be left outer joins, but you can rewrite your statement like this. This way will work regardless of the number of fields in each of the tables (requirement for the union), and if they are named differently then you can access the appropriate field by name.
DECLARE #var int
SET #var=1
select tbl.*, tblA.ShippingName, tblB.BillingName, tblC.OtherName
from tbl
left outer join tblA on tbl.id = tblA.id and #var = 1
left outer join tblB on tbl.id = tblB.id and #var = 2
left outer join tblC on tbl.id = tblC.id and #var = 3
I have a stored procedure which uses different tables for a join based on an input parameter. Currently I have to write the SQL query twice (with only the table name difference). Is it possible to combine them so I do not have to repeat SQL query logic twice?
Current code:
CREATE PROCEDURE SampleProc
#Condition BIT
AS
IF #Condition = 0
BEGIN
SELECT *
FROM TableA1 A /* Use TableA1 instead of TableA2 */
INNER JOIN TableB B ON A.Id = B.Id /* The rest of the query remains the same */
/* Inner Join some more complex join logic */
END
ELSE
BEGIN
SELECT *
FROM TableA2 A /* Use TableA2 instead of TableA1 */
INNER JOIN TableB B ON A.Id = B.Id /* The rest of the query remains the same */
/* Inner Join some more complex join logic */
END
END
One of the possible ways is to store TableA1 / TableA2 data to a temp table first and use the temp table to join inside a complex query. Is there any better way?
If the two tables have the same structure (as implied by the temp table comment), you can do:
select . . .
from ((select a.* from tablea1 a where #condition = 0
) union all
(select a.* from tablea2 a where #condition <> 0
)
) a inner join
b
Another alternative is dynamic SQL, but that can be tricky to maintain -- because the code looks like a string.
Sometimes, you can do this with a left join as well:
select b.*, coalesce(a1.col, a2.col) as col
from b left join
tablea1 a1
on a1.id = b.id and #condition = 0 left join
tablea2 a2
on a2.id = b.id and #condition <> 0
where a1.id is not null or a2.id is not null
. . .
Although this should have good performance, it has the downside that all references to a columns need to use coalesce().
if TableA1 and TableA2 have same columns, try this
SELECT
*
From
( select
*
from
TableA1
where
#Condition = 0
union all
select
*
from
TableA2
where
#Condition != 0) as A
INNER JOIN
TableB B
On
A.Id =B.Id
I have two table in my same DB
create table a(gr_code nvarchar, code int)
insert into a values('1',100),('0',200),('1',200),('0',100)
create table b(gr_code nvarchar, code int)
insert into b values('1',100),('0',200)
find the code in table A which does not have in table B for particular gr_code
expected result:
gr_code code
1 200
0 100
It's quite simple using the clause exists
select *
from a
where not exists (select *
from b
where b.gr_code = a.gr_code and
b.code = a.code)
This returns the result on your sample.
Use LEFT JOIN
select a.*
from a
left join b on a.gr_code = b.gr_code and a.code = b.code
where b.gr_code is null
You can use LEFT JOIN like this:
SELECT a.*
FROM a
LEFT JOIN b on a.gr_code = b.gr_code
WHERE b.gr_code IS NULL
You can use EXCEPT like so:
select *
from a
except
select *
from b
This question already has answers here:
Difference between "on .. and" and "on .. where" in SQL Left Join? [duplicate]
(6 answers)
Closed 8 years ago.
I normally don't use an AND in the same line as ON when performing a LEFT JOIN, as I have faced issues in the past. I rather prefer to not go into this pickle and instead put any additional condition in a WHERE clause, which is reliable. But today, out of curiousity, I would love to put this question forward and be clear once and for all.
Question: What really goes on when in a LEFT JOIN when I use the "exta" conditions? Why doesn't it behave in the same manner as WHERE?
SAMPLE QUERIES
create table #a
(
id int,
name varchar(3)
)
create table #b
(
id int,
name varchar(3)
)
insert into #a
select 1, 'abc'
union
select 2, 'def'
union
select 3, 'ghi'
insert into #b
select 1, 'abc'
union
select 2, 'def'
select * from #a a left join #b b on a.id = b.id
where a.id = 3
select * from #a a left join #b b on a.id = b.id
and a.id = 3
This version filters on a.id:
select *
from #a a left join
#b b
on a.id = b.id
where a.id = 3
This version does not filter on a.id:
select *
from #a a left join
#b b
on a.id = b.id and a.id = 3;
Why not? Go to the definition of the left join. It takes all rows from the first table, regardless of whether the on clause evaluates to true, false, or NULL. So, filters on the first table have no impact in a left join.
Filters on the first table should be in the where clause. Filters on the second table should be in the on clause.
You can put conditions on the "right tables" columns in the ON clause.
I have 2 tables say A and B, and I want to do a join on them.
Table A will always have records in it.
When table B has rows in it, I want the query to turn all the rows in which table A and table B matches. (i.e. behave like inner join)
However, if table B is empty, I'd like to return everything from table A.
Is this possible to do in 1 query?
Thanks.
Yes, for results like this, use LEFT JOIN.
Basically what INNER JOIN does is it only returns row where it has atleast one match on the other table. The LEFT JOIN, on the other hand, returns all records on the left hand side table whether it has not match on the other table.
To further gain more knowledge about joins, kindly visit the link below:
Visual Representation of SQL Joins
I came across the same question and, as it was never answered, I post a solution given to this problem somewhere else in case it helps someone in the future.
See the source.
select *
from TableA as a
left join TableB as b
on b.A_Id = a.A_Id
where
b.A_Id is not null or
not exists (select top 1 A_Id from TableB)
Here is another one, but you need to add one "null" row to table B if it's empty
-- In case B is empty
Insert into TableB (col1,col2) values (null,null)
select *
from TableA as a inner join TableB as b
on
b.A_Id = a.A_Id
or b.A_Id is null
I would use an if-else block to solve it like below:
if (select count(*) from tableB) > 0
begin
Select * from TableA a Inner Join TableB b on a.ID = b.A_ID
end
else
begin
Select * from TableA
end
Try This
SELECT t1.* FROM table1 AS t1 INNER JOIN table2 AS t2 ON t1.something = t2.someotherthing UNION SELECT * FROM table1 WHERE something = somethingelse;
This is solution:
CREATE TABLE MyData(Id INT, Something VARCHAR(10), OwnerId INT);
CREATE TABLE OwnerFilter(OwnerId INT);
SELECT *
FROM
(SELECT NULL AS Gr) AS Dummy
LEFT JOIN OwnerFilter F ON (1 = 1)
JOIN MyData D ON (F.OwnerId IS NULL OR D.OwnerId = F.OwnerId);
Link to sqlfiddle: http://sqlfiddle.com/#!6/0f9d9/7
I did the following:
DECLARE #TableB TABLE (id INT)
-- INSERT INTO #TableB
-- VALUES (some ids to filter by)
SELECT TOP 10 *
FROM [TableA] A
LEFT JOIN #TableB B
ON A.ID = B.id
WHERE B.id IS NOT NULL
OR iif(exists(SELECT *
FROM TableB), 1, 0) = 0
Now:
If TableB is empty (leave the commented lines commented) you'll get the top 10.
If TableB has some ids in it, you'll only join by those.
I do not know how efficient this is. Comments are welcome.
Maybe use a CTE
;WITH ctetable(
Select * from TableA
)
IF(EXISTS(SELECT 1 FROM TableB))
BEGIN
Select * from ctetable
Inner join TableB
END
ELSE
BEGIN
Select * from ctetable
END
or dynamic SQL
DECLARE #Query NVARCHAR(max);
SET #QUERY = 'Select * FROM TableA';
IF(EXISTS(SELECT 1 FROM TableB))
BEGIN
SET #QUERY = CONCAT(#QUERY,' INNER JOIN TableB');
END
EXEC sp_executesql #Query