SELECT * FROM Table A LEFT JOIN TABLE B LEFT JOIN TABLE C
From the snippet above, TABLE C will left join into (TABLE B) or (data from TABLE A LEFT JOIN TABLE B) or (TABLE A)?
TABLE C will left join into 1. (TABLE B) or 2. (data from TABLE A LEFT JOIN
TABLE B) or 3. (TABLE A)?
The second. But The join condition will help you to understand more.
You can write:
SELECT *
FROM Table A
LEFT JOIN TABLE B ON (A.id = B.id)
LEFT JOIN TABLE C ON (A.ID = C.ID)
But you are able to:
SELECT *
FROM Table A
LEFT JOIN TABLE B ON (A.id = B.id)
LEFT JOIN TABLE C ON (A.id = C.id and B.code = C.code)
So, you can join on every field from previous tables and you join on "the result" (though the engine may choose its way to get the result) of the previous joins.
Think at left join as non-commutative operation (A left join B is not the same as B left join A) So, the order is important and C will be left joined at the previous joined tables.
The Oracle documentation is quite specific about how the joins are processed:
To execute a join of three or more tables, Oracle first joins two of
the tables based on the join conditions comparing their columns and
then joins the result to another table based on join conditions
containing columns of the joined tables and the new table. Oracle
continues this process until all tables are joined into the result.
This is the logic approach to handling the joins and is consistent with the ANSI standard (in other words, all database engines process the joins in order).
However, when the query is actually executed, the optimizer may choose to run the joins in a different order. The result needs to be logically the same as processing the joins in the order given in the query.
Also, the join conditions may cause some unexpected conditions to arise. So if you have:
from A left outer join
B
on A.id = B.id left outer join
C
on B.id = C.id
Then, you might have the condition where A and C each have a row with a particular id, but B does not. With this formulation, you will not see the row in C because it is joining to NULL. So, be careful with join conditions on left outer join, particularly when joining to a table other than the first table in the chain.
You need to mentioned the column name properly in order to run the query. Let´s say if you are using:
SELECT *
FROM Table A
LEFT JOIN TABLE B ON (A.id = B.id)
LEFT JOIN TABLE C ON (A.id = C.id and B.code = C.code)
Then you may get the following error:
ORA-00933:SQL command not properly ended.
So to avoid it you can try:
SELECT A.id as "Id_from_A", B.code as "Code_from_B"
FROM Table A
LEFT JOIN TABLE B ON (A.id = B.id)
LEFT JOIN TABLE C ON (A.id = C.id and B.code = C.code)
Thanks
Related
I have four tables, that have a key in common.
three of the four tables are small subsets of the fourth,(master).
I want to join the tables such that only the output only contains records from the master table, that are on any of other fourth:
as an example:
My end result should look like this:
My problem is that joins I'm using, are giving me only the records that are common to all tables.
or records that are common to only one of the tables and the master.
Any help on formulating the correct join would be awesome!
Three left joins will produce the result you want. For example:
select a.*, b.color, c.size, d.weight
from a
left join b on b.id = a.id
left join c on c.id = a.id
left join d on d.id = a.id
where b.id is not null or c.id is not null or d.id is not null
EDIT: Added WHERE clause above as requested.
When doing an outer join, is it the order of the tables that matter or the order of the ON clause?
For example is
FROM TABLEA A
LEFT JOIN TABLEB B ON A.id = B.id
the same as
FROM TABLEA A
LEFT JOIN TABLEB B ON B.id = A.id
What about if you have multiple tables? Is it a LEFT JOIN if the first table out of many is the one you want all rows from regardless of the ON clause?
For example,
FROM TABLEA A
LEFT JOIN TABLEB B ON A.id = B.id
LEFT JOIN TABLEC C ON C.ID = A.ID
Does it take all the rows from TABLEA because it is to the left in the table list or the rows from C because it is on the left in the ON clause?
LEFT JOIN means take the table on the left (the first one specified), and join the rows from the table on the right (the second one specified). It will join them up based on the ON condition being true. Since the ON condition just needs to be true, the way it is written doesn't matter at all, it's just an expression that is evaluated.
LEFT JOIN ensures that every row from the table on the left is retained, and joined with NULLs if there is no row to join up to it from the table on the right. So that means that order of the tables is certainly significant.
If the table ordering was reversed, and there were only two tables, a RIGHT JOIN would have the same effect (i.e. keep the rows from the second table specified).
FROM TABLEA A LEFT JOIN TABLEB B ON A.id=B.id LEFT JOIN TABLEC C ON C.ID=A.ID
FROM TABLEA A LEFT JOIN TABLEB B ON B.id=A.id LEFT JOIN TABLEC C ON A.ID=C.ID
These both would return the same results.
FROM TABLEA A LEFT JOIN TABLEB B ON B.id=A.id LEFT JOIN TABLEC C ON B.ID=C.ID
This might or might not return the same results depending on what data is actually in table b and table C because table C is related to table b not directly related to table A. Personally I would always treat this as being different by definition than the first set of joins and that if is is the same that is accidental at this point in time.
When writing multiple joins especially when you have Outer joins, I personally find it helpful to start with the parent table (that usually being the one you want on the left side of the join) first and add any other other inner join tables before doing the left joins. If I have child and grandchild tables (vice multiple child tables), then I try to do in descending order of parent, child, grandchild to be clear what is related to what.
Can sombody Explains me about joins?
Inner join selects common data based on where condition.
Left outer join selects all data from left irrespective of common but takes common data from right table and vice versa for Right outer.
I know the basics but question stays when it comes to join for than 5, 8, 10 tables.
Suppose I have 10 tables to join. If I have inner join with the first 5 tables and now try to apply a left join with the 6th table, now how the query will work?
I mean to say now the result set of first 5 tables will be taken as left table and the 6th one will be considerded as Right table? Or only Fifth table will be considered as left and 6th as right? Please help me regarding this.
When joining multiple tables the output of each join logically forms a virtual table that goes into the next join.
So in the example in your question the composite result of joining the first 5 tables would be treated as the left hand table.
See Itzik Ben-Gan's Logical Query Processing Poster for more about this.
The virtual tables involved in the joins can be controlled by positioning the ON clause. For example
SELECT *
FROM T1
INNER JOIN T2
ON T2.C = T1.C
INNER JOIN T3
LEFT JOIN T4
ON T4.C = T3.C
ON T3.C = T2.C
is equivalent to (T1 Inner Join T2) Inner Join (T3 Left Join T4)
It's helpful to think of JOIN's in sequence, so the former is correct.
SELECT *
FROM a
INNER JOIN b ON b.a = a.id
INNER JOIN c ON c.b = b.id
LEFT JOIN d ON d.c = c.id
LEFT JOIN e ON e.d = d.id
Would be all the fields from a and b and c where all the ON criteria match, plus the values from d where its criteria match plus all the contents of e where all its criteria match.
I know RIGHT JOIN is perfectly acceptable, but I've found in my experience that it's unnecessary - I almost always just join things from left to right.
> Simple INNER JOIN VIEW code...
CREATE VIEW room_view
AS SELECT a.*,b.*
FROM j4_booking a INNER JOIN j4_scheduling b
on a.room_id = b.room_id;
You can apply join like this..
select a.*,b.*,c.*,d.*,e.*
from [DatabaseName].[Table_a] a
INNER JOIN [DatabaseName].[Table_b] b ON a.id = b.id
INNER JOIN [DatabaseName].[Table_c] c ON b.id=c.id
INNER JOIN [DatabaseName].[Table_d] d on c.id=d.id
INNER JOIN [DatabaseName].[Table_e] e on d.id=e.id where a.con=5 and
b.con=6
Here, at place of a.* and in where condition, you can show column(filed) which you like and according condition in where condition. You can insert more table and database as per your choice. But mind that you need to mention database name and alias if you work in different database.
Just tried the following from the Example DataBase given in W3School. Worked Fine for me.
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate, Products.ProductName, Products.ProductID
FROM Orders
INNER JOIN Products
INNER JOIN Customers
ON Orders.CustomerID=Customers.CustomerID;
Join used to combine rows from two or more tables, based on a related column between them. This example from Adventure works:
SELECT a.[EmailAddress],b.[FirstName],b.[LastName],c.[PhoneNumber],d.[Name]
FROM [Person].[EmailAddress] a
INNER JOIN [Person].[Person] b
ON a.BusinessEntityID = b.BusinessEntityID
INNER JOIN [Person].[PersonPhone] c
ON b.BusinessEntityID = c.BusinessEntityID
INNER JOIN [Person].[PhoneNumberType] d
ON c.phoneNumberTypeID = d.phoneNumberTypeID
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.
Suppose I have three tables, A, B and C.
I did Join on table A and table B, now I wanted to do Join on result AB and table c.
Do I need to create view and then do Join or need to do it by Nested query?
You don't say which DB you're using, so the syntax could be wrong, but multi-table joins aren't any different, really, than joining two tables:
SELECT ...
FROM a
JOIN b ON ...
JOIN c ON ...
JOIN d ON ...
No, you will do it as follows
SELECT *
FROM A [INNER/LEFT/RIGHT/OUTER] JOIN
B ON [a/b].IDCols = [a/b].IDCols [INNER/LEFT/RIGHT/OUTER] JOIN
C ON [a/b/c].IDCols = [a/b/c].IDCols
The specific joins (INNER/LEFT/RIGT/OUTER) will depend on what your requirements are.
Have a look at Introduction to JOINs – Basic of JOINs for an overview
The criteria for the JOIN ONs will also depend on how the tables relate to one another.
You can either use something like this:
select *
from A, B, C
where A.id = B.id
and A.id = C.id
or you can use something like this:
select *
from A INNER JOIN B ON (A.id = B.id)
INNER JOIN C ON (A.id = C.id)
How you join will of course depend on how the tables relate to each other, so what's the primary key and foreign key of A,B,C.
You might also use OUTER JOIN instead of INNER JOIN, depending on our data.