Hello I am Confused with three scenarios which commonly every one use in almost every project.
I wanted to Know which one of these will be Efficient accordinng to - Less Time Complexity - Efficiency - effectiveness
TableA (userid ,username, email , phone)
TableB (username,TestField)
.
Case 1
select email, TestField from TableA , TableB
where TableA.username = TableB.username and
TableB.username = 'ABC'
group by email, TestField
Case 2
select email, TestField from TableA
inner join TableB on TableB.username = 'ABC'
Case 3
declare #uname nvarchar(20);
set #uname = 'ABC';
declare #Email nvarchar(20);
select #Email= email from TableA where username = #uname;
select #Email as email , TestField from TableB
where username = #uname
Case 2 will give you a different output anyway, as you are not joining TableA and TableB in any way so you get a Cartesian product.
Since all of a sudden email came up, you will need a join in case 1:
In Case 1 you can simply rewrite the query to
SELECT DISTINCT A.Email , B.TestField
FROM TableA A join TableB B on A.username = B.Username
WHERE B.username = 'ABC'
Which is more readable and easier to maintain as you do not ave a superfluous GROUP BY clause.
In Case 3 you have userId in your where clause, which is not even in your tableB according to your post.
In general, for maintainability and readibility:
Use explicit joins
SELECT * FROM A JOIN B ON A.id = B.id
is preferable over
SELECT * FROM A, B WHERE A.id = B.id
And use DISTINCT when you want distinct values, instead of GROUP BY over all columns:
SELECT DISTINCT a, b, b FROM TABLE
is preferable over
SELECT a, b, c FROM TABLE GROUP BY a, b, c
Most database experts will tell you that cross products are evil and to be avoided. Your first example would work just fine. It is an implicit inner join.
Your second example is syntactically incorrect. I suspect you'd get an error from MSSQL Server Manager. What you probably meant was:
select a.email, b.TestField
from TableA a inner join TableB b
on (b.username = a.username)
where b.username = 'ABC'
Your first example will probably be the more efficient, since MSSQL Server is smart enough to do the projection on TableB.username before doing the join. I'm not so certain that this would be the case in the above version of case 2.
To be sure you could do it like this:
select a.email, b.TestField
from TableA a inner join
(select * from TableB where TableB.username = 'ABC') b
on (b.username = a.username)
where b.username = 'ABC'
Hope that helps.
Related
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
I have two tables
Table A
-------
ID
ProductName
Table B
-------
ID
ProductID
Size
I want to join these two tables
SELECT * FROM
(SELECT * FROM A)
LEFT JOIN
(SELECT * FROM B)
ON A.ID = B.ProductID
This is easy, I will get all rows from A multiplied by rows matched in B, and NULL fields if there is no match.
But here comes the tricky question, how can I get all rows from A with NULL fields for table B, even if there is a match, so I get an extra line with NULL values plus all the matches?
SELECT A.*
, B3.ID
, B3.ProductID
, B3.Size
FROM A
LEFT JOIN
(
SELECT ProductID as MatchID
, ID
, ProductID
, Size
FROM B
UNION ALL
SELECT ID
, null
, null
, null
FROM A A2
) B3
ON A.ID = B3.MatchID
Live example at SQL Fiddle.
Instead of using UNION ALL in a subquery as suggested by others, you could also (and I would) use UNION ALL at the outer level, which keeps the query simpler:
SELECT A.ID, A.ProductName, B.ID, B.Size
FROM A
INNER JOIN B
ON B.ProductID = A.ID
UNION ALL
SELECT A.ID, A.ProductName, NULL, NULL
FROM A
Since every join is going to be successful, we can switch to a full/inner join:
SELECT
*
FROM
A
INNER JOIN
(SELECT ID,ProductID,Size FROM B
UNION ALL
SELECT NULL,ID,NULL FROM A) B
ON
A.ID = B.ProductID
Now would be a very good time to switch to naming columns explicitly, rather than using SELECT *
Or, if, as per #Andomar's comment, you need all of the B columns to be NULL:
SELECT
A.ID,A.ProductName,
B.ID,B.ProductID,B.Size
FROM
A
INNER JOIN
(SELECT ID,ProductID,Size,ProductID as MatchID FROM B
UNION ALL
SELECT NULL,NULL,NULL,ID FROM A) B
ON
A.ID = B.MatchID
I have a long stored procedure and I would like to make a slight modification to the procedure without having to create a new one(for maintenance purposes).
Is it possible to use a IF or CASE in the FROM statement of the select statement to join other tables?
Like this:
from tableA a
join tableB b a.indexed = c.indexed
IF #Param='Y'
BEGIN
join tableC c a.indexed = c.indexed
END
It didn't seem to work for me. But I am wondering if this is even possible and/or if this even makes sense to do.
Thanks.
No, it is not possible. You can only accomplish this through the use of dynamic SQL.
The Curse and Blessings of Dynamic SQL
An Intro to Dynamic SQL
I would not advise using Dynamic SQL, there are most likely better ways to perform this operation but you would have to provide more info.
You can achieve something like it if you have a left outer join
Consider
declare #param bit = 1
select a.*, b.*, c.* from a
inner join b on a.id = b.a_id
left outer join c on b.id = c.b_id and #param = 1
This will return all columns from a, b, c.
Now try with
declare #param bit = 0
This will return all columns from a and b, and nulls for columns of c.
It won't work if both joins are inner.
No this is not possible. Your best bet would probably be to select from both tables and only include the data your care about. If you provide an example of what you are trying to do I can provide a better answer.
Attempt at an example:
SELECT t1.id, COALESCE(t2.name, t3.name)
FROM Table1 as t1
LEFT JOIN Table2 as t2
ON t1.id = t2.id
LEFT JOIN Table2 as t3
ON t1.id = t3.id
While what you proposed is not possible, you can play with your where conditions:
from tableA a
inner join tableB b ON a.indexed = c.indexed
left join tableC c ON a.indexed = c.indexed AND 1 = CASE #Param WHEN 'Y' THEN 1 ELSE 0 END
More performant would be to just doing a big
IF #Param='Y' THEN
from tableA a
inner join tableB b ON a.indexed = c.indexed
ELSE
from tableA a
inner join tableB b ON a.indexed = c.indexed
left join tableC c ON a.indexed = c.indexed
You haven't revealed you SELECT clause. The essence of what you want is as follows:
SELECT indexed
FROM tableA
INTERSECT
SELECT indexed
FROM tableB
INTERSECT
SELECT indexed
FROM tableC
WHERE #Param = 'Y'
Then use this table expression as dictated by your SELECT clause e.g. say you only want to project tableA:
WITH T
AS
(
SELECT indexed
FROM tableA
INTERSECT
SELECT indexed
FROM tableB
INTERSECT
SELECT indexed
FROM tableC
WHERE #Param = 'Y'
)
SELECT *
FROM tableA
WHERE indexed IN ( SELECT indexed FROM T );
I am trying to build a select query that will essentially left join two tables and display one column from each table as a single column.
the table structure is similar to:
table a:
id, email
table b:
id, tablea_id, email
I am trying to get a single column of email and email (with no dupes or nulls ideally).
ideal results would be:
one#one.com
two#two.com
three#three.com
and the email address that is returned could be from either a or b.
Maybe a union is what would work best, but I not able to figure out how to do a union on the second table based on the id of the first table.
When searching for a solution, perhaps my wording is bad, but I can't find any examples.
thanks for any help.
It seems that you want something on the lines of this:
SELECT email
FROM TableA
UNION
SELECT B.email
FROM TableA A
JOIN TableB B
ON A.id = B.TableA_id
If you simply want a unique list of email addresses from either table you can do:
Select email
From TableA
Union
Select email
From TableB
If you are looking for a unique list of email addresses from Table B and those from Table A that exist in Table B, then you can do:
Select TableA.email
From TableA
Join TableB
On TableB.TableA_id = TableA.id
Union
Select email
From TableB
If, per your comments, you need all rows from Table A and only rows from Table B where they exist in Table A:
Select email
From Table A
Union
Select TableB.email
From TableB
Join TableA
On TableA.id = TableB.TableA_id
How about this:
SELECT DISTINCT a.email+ ' ' + b.email FROM tableA a LEFT JOIN tableB b on a.Id = b.tablea_Id WHERE b.email IS NOT NULL
This could help:
SELECT
id, email
FROM
a
UNION
(
SELECT
a.id AS id, b.email AS email
FROM
b
INNER JOIN
a
ON
a.id = b.tablea_id
) b
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'