SQL condition with NULL - sql

I have the following working select:
SELECT
TableA.FullName
FROM
TableA,
TableB
WHERE
TableA.Contact = TableB.Contact
But I have some lines in TableB that have NULL in the TableB.Contact column, and I would like to show it. I tried:
WHERE
(TableA.Contact = TableB.Contact OR TableB.Contact IS NULL)
Without lucky.

You need to use a LEFT JOIN
SELECT
a.FullName, a.Contact, b.Contact
FROM TableA a
LEFT JOIN TableB b ON b.Contact = a.Contact
WHERE
(a.Contact = b.Contact OR b.Contact IS NULL)
The WHERE clause is redundant here - is does the same as the LEFT JOIN

You are using an outdated JOIN format. Try changing it to this:
SELECT column1, column2, column3 FROM
TableA as a
LEFT JOIN TableB as b
ON a.contact = b.contact
Using a left join will pull all records from A regardless if there is a match in B. This is a good explanation of how the various joins work:
http://blog.codinghorror.com/a-visual-explanation-of-sql-joins/
Also, notice that I am specifically pulling only the columns I want instead of *. This is a good habit to get into with SQL.
UPDATE:
After your comments, it seems like you are looking for a FULL OUTER JOIN?
SELECT column1, column2
FROM TableA as a
FULL OUTER JOIN TableB as b
ON a.contact = b.contact

Try changing the join condition:
SELECT *
FROM TableA JOIN
TableB
ON TableA.Contact = TableB.Contact OR
TableA.Contact IS NULL AND TableB.Contact IS NULL;
A left outer join won't do exactly what you want. It keeps all rows in TableA, but doesn't check for matching values.
For performance reasons, the following might work better:
SELECT *
FROM TableA JOIN
TableB
ON TableA.Contact = TableB.Contact
UNION ALL
SELECT *
FROM TableA CROSS JOIN
TableB
WHERE TableA.Contact IS NULL AND TableB.Contact IS NULL;

Related

Hive - Where and OR clause error

Hi I am trying to run this query in Hive, but get the error 10249 (Unsupported query expression - only 1 subquery is supported...)
select count(*) from
(
select * from tableA
union all
select * from tableB
) a
where a.field1 in (select fieldA in tableC)
or a.field2 in (select fieldA in tableC)
or a.field3 in (select fieldA in tableC);
Would anybody know how I can write this so that Hive supports this query (works fine in SQL server)
Since you do not need fields from tableC, you can use left semi join instead of in:
select count(*) from
(
select * from tableA
union all
select * from tableB
) a
left semi join tableC c1 on a.field1=c1.fieldA
left semi join tableC c2 on a.field2=c2.fieldA
left semi join tableC c3 on a.field3=c3.fieldA
;
left semi join is half join, the result set contains fields only from one of joined tables, only joined rows returned, similar to inner join but will not create duplicates if right table contains multiple matching rows.
LEFT SEMI JOIN implements the uncorrelated IN/EXISTS subquery semantics in an efficient way.
Covert you sub query in CTE , left join and use or condition in where clause.
IE.
with temp as
(
select * from tableA
union all
select * from tableB
)
select COUNT(a.*)
from temp a left join tableC a1 on a.field1 =a1.fieldA
left join tableC a2 on a.field2 =a2.fieldA
left join tableC a3 on a.field3 =a3.fieldA
where a1.fieldA is not null
or a3.fieldA is not null
or a3.fieldA is not null

How to add conditional left or inner join based on parameter in SQL?

I am trying to add conditional left or inner join based on parameter in my select statement.
For example :
I have TableA and TableB, and I have a parameter called #test,
I wanna do something like this :
SELECT * FROM TableA
IF(#test ='')
INNER JOIN TableB
ELSE
LEFT OUTER JOIN TableB
IS that possible in SQL Server 2012 ?
You cannot change the kind of the JOIN based on a parameter, but you can filter out the rows with NULLs in an outer join.
Suppose that TableB is joined to TableA on b.a_id = a.id. Then you could write this query:
SELECT * FROM TableA a
LEFT OUTER JOIN TableB b
ON b.a_id = a.id
WHERE #test != '' OR b.a_id IS NOT NULL
Not exactly (except using dynamic SQL), but you can do this in the where clause:
SELECT *
FROM TableA LEFT OUTER JOIN
TableB
ON . . .
WHERE (#test = '' and TableB.id is not null) or (#test <> '')

how to do a join using a conditional ON

I would like to do as follows:
Select * from
table1 a
inner join table2 b
on
a.id = b.id
if (some condition is met) // Now it gets interesting!
begin
and a.name = b.name
end
Obviously, this doesn't work.
How can this best be accomplished?
Thanks Stackers!
Why can't you just put the condition in the WHERE-clause?
Generally, you would make a conditional join something like this:
Select *
from table1 a
inner join table2 b
on (a.conditional_field = 1 and a.id = b.id)
or (a.conditional_field = 2 and a.id2 = b.id2)
The important thing to note here is that makes the join condition optional, not the join itself. If you're looking to make the join itself conditional, that's what outer joins are for:
Select *
from table1 a
left outer join table2 b
on a.id = b.id
The first query will return all matching rows from either condition is true. The second query will unconditionally return all rows from table1 and only those rows from table2 where the condition is true.
I would use something like this:
SELECT * FROM table1 a
JOIN table2 b ON (a.id = b.id)
WHERE NOT ( == your condition here == ) OR a.name = b.name
If you really want to put it in the join condition, you could do something like this:
SELECT * FROM table1 a
JOIN table2 b ON (a.id = b.id AND (NOT ( == your condition here == ) OR a.name = b.name))
but I think the first form is more clear.
EDIT: as #James Curtis noted in the comments:
it is important to note that the option to put the condition in the
WHERE clause is only valid for an INNER JOIN, for an outer join you
may eliminate rows.

In SQL, What's the difference a ON condition following a Join vs at the end of multiple JOINS

I have been having a hard time googling an answer for this, but....
can someone explain to me the difference between putting the ON condition of a JOIN with the the JOIN itself vs putting the ON at the end of all the other JOINs.
here is an example http://sqlfiddle.com/#!3/e0a0f/3
CREATE TABLE TableA (Email VARCHAR(100), SomeNameA VARCHAR(100))
CREATE TABLE Tableb (Email VARCHAR(100), SomeNameB VARCHAR(100))
CREATE TABLE Tablec (Email VARCHAR(100), SomeNameC VARCHAR(100))
INSERT INTO TableA SELECT 'joe#test.com', 'JoeA'
INSERT INTO TableA SELECT 'jan#test.com', 'JaneA'
INSERT INTO TableA SELECT 'dave#test.com', 'DaveA'
INSERT INTO TableB SELECT 'joe#test.com', 'JoeB'
INSERT INTO TableB SELECT 'dave#test.com', 'DaveB'
INSERT INTO TableC SELECT 'joe#test.com', 'JoeC'
INSERT INTO TableC SELECT 'dave#test.com', 'DaveC'
SELECT TOP 2 a.*,
b.*,
c.*
FROM TableA a
LEFT OUTER JOIN TableB b
ON a.email = b.email
INNER JOIN TableC c
ON c.Email = b.email;
SELECT TOP 2 a.*,
b.*,
c.*
FROM TableA a
LEFT OUTER JOIN TableB b
INNER JOIN TableC c
ON c.Email = b.email
ON a.email = b.email;
I don't understand why these two SELECT statements produce different results.
What matters is orders of joins. Treat your expressions as if every join produced temporary "virtual" table.
So when you write
FROM TableA a
LEFT OUTER JOIN TableB b ON a.email = b.email
INNER JOIN TableC c ON c.Email = b.email ;
then order is as follows:
TableA is left joined to TableB producing temporary relation V1
V1 is inner joined to TableC.
Meanhwile when you write:
FROM TableA a
LEFT OUTER JOIN TableB b
INNER JOIN TableC c ON c.Email = b.email ON a.email = b.email;
then order is as follows:
TableB is inner joined to TableC producing temporary relation V1.
TableA is left joined to V1.
Thus results are different. It is generally recommended to use parenthesis in such situations to improve readability of the query:
FROM TableA a
LEFT OUTER JOIN
(TableB b INNER JOIN TableC c ON c.Email = b.email)
ON a.email = b.email;
In your second example, the part ON a.email = b.email belongs to the LEFT JOIN.
If written like this, it means the following:
INNER JOIN TableC with TableB and LEFT OUTER JOIN the result with TableA.
The result will be all rows from TableA joined with those rows from TableB that also have an entry in TableC.
The first example means the following:
LEFT OUTER JOIN TableB with TableA and INNER JOIN TableC with the result. This is equivalent to using an INNER JOIN for TableB.
Explanation: When you LEFT OUTER JOIN TableA with TableB you will get all rows from TableA and for matching rows in TableB you will get that data, too. In your result set you will have rows with b.email = NULL and this will now be INNER JOINed with TableC. As long as there is no entry in TableC with email = NULL you will get the results you observed.

SELECT * FROM tableA, tableB WHERE Conditions [+]

I have the following query
SELECT *
FROM tableA, tableB
WHERE Conditions [+]
What does this keyword Conditions[+] Stands for?
How this query behaves as a outer join?
That is old Oracle Join syntax.
SELECT *
FROM tableA, tableB
WHERE Conditions [+] -- this should be tableA (+) = tableB
The positioning of the + sign denotes the JOIN syntax.
If you query was:
SELECT *
FROM tableA, tableB
WHERE tableA.id (+) = tableB.Id
Then it would be showing a RIGHT OUTER JOIN so the equivalent is:
SELECT *
FROM tableA
RIGHT OUTER JOIN tableB
ON tableB.id = tableA.Id
If the + sign was on the other side then it would be a LEFT OUTER JOIN
SELECT *
FROM tableA, tableB
WHERE tableA.id = tableB.Id (+)
is equivalent to
SELECT *
FROM tableA
LEFT OUTER JOIN tableB
ON tableA.id = tableB.Id
I would advise using standard join syntax though.
If you do not specify a + sign then it will be interpreted as an INNER JOIN
SELECT *
FROM tableA, tableB
WHERE tableA = tableB
it's equivalent is:
SELECT *
FROM tableA
INNER JOIN tableB
ON tableA.id = tableB.id
A FULL OUTER JOIN would be written using two SELECT statements and a UNION:
SELECT *
FROM tableA, tableB
WHERE tableA.id = tableB.Id (+)
UNION
SELECT *
FROM tableA, tableB
WHERE tableA.id (+) = tableB.Id
It's equivalent is:
SELECT *
FROM tableA
FULL OUTER JOIN tableB
ON tableA.id = tableB.id
Here is a tutorial that explains a lot of these:
Old Outer Join Syntax
It is not important how that behaves. You should use the standard syntax for outer joins:
select *
from tableA left outer join
tableB
on . . .
The "(+)" syntax was introduced by Oracle before the standard syntax, and it is highly out of date.
'Conditions' here just means what you're using to filter all this data.
LIke here's an example:
SELECT *
FROM tableA, tableB
WHERE Name like '%Bob%'
would return names that have "Bob" anywhere inside.
About outer joins, actually you'd use that in the FROM clause:
So maybe
SELECT *
FROM tableA ta
OUTER JOIN tableB tb
ON ta.name = tb.name
WHERE ta.age <> 10
and there where here is optional, by the way
I hate to just copy & paste an answer, but this sort of thing can be found pretty easily if you do a little searching...
An outer join returns rows for one table, even when there are no
matching rows in the other. You specify an outer join in Oracle by
placing a plus sign (+) in parentheses following the column names from
the optional table in your WHERE clause. For example:
SELECT ut.table_name, uc.constraint_name
FROM user_tables ut, user_constraints uc
WHERE ut.table_name = uc.table_name(+);
The (+) after uc.table_name makes the user_constraint table optional.
The query returns all tables, and where there are no corresponding
constraint records, Oracle supplies a null in the constraint name
column.