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.
Related
Are next two queries going to return same result set?
SELECT * FROM tableA a
JOIN tableB b
ON a.id = b.id
WHERE a.id = '5'
--------------------------------
SELECT * FROM tableA a
JOIN tableb b
ON a.id = b.id
WHERE b.id = '5'
Also, will answer be different if LEFT JOIN is used instead of JOIN?
As written, they will return the same result.
The two will not necessarily return the same result with a left join.
Yes the result will be the same.
With a left join you will get every dataset of both table who got a ID.
With a join (Inner Join) you will get only the dataset's who a.id = b.id.
This site will explain you how to join https://www.w3schools.com/sql/sql_join.asp
Yes they will. A simple join works like an inner join by default. It checks for instances where the item you're joining on exist on both tables. Since you're joining on where a.id=b.id the results will be the same.
If you change the type of join to a left, the results will include all a.id's regardless of whether they are equal to 5.
I have the following scenario on a SQL Server 2008 R2:
The following queries returns :
select * from TableA where ID = '123'; -- 1 rows
select * from TableB where ID = '123'; -- 5 rows
select * from TableC where ID = '123'; -- 0 rows
When joining these tables the following way, it returns 1 row
SELECT A.ID
FROM TableA A
INNER JOIN ( SELECT DISTINCT ID
FROM TableB ) AS D
ON D.ID = A.ID
INNER JOIN TableC C
ON A.ID = C.ID
ORDER BY A.ID
But, when switching the inner joins order it does not returns any row
SELECT A.ID
FROM TableA A
INNER JOIN TableC C
ON A.ID = C.ID
INNER JOIN ( SELECT DISTINCT ID
FROM TableB ) AS D
ON D.ID = A.ID
ORDER BY A.ID
Can this be possible?
Print Screen:
For inner joins, the order of the join operations does not affect the query (it can affect the ordering of the rows and columns, but the same data is returned).
In this case, the result set is a subset of the Cartesian product of all the tables. The ordering doesn't matter.
The order can and does matter for outer joins.
In your case, one of the tables is empty. So, the Cartesian product is empty and the result set is empty. It is that simple.
As Gordon mentioned, for inner joins the order of joins doesn't matter, whereas it does matter when there's at least one outer join involved; however, in your case, none of this is pertinent as you are inner joining 3 tables, one of which will return zero rows - hence all combinations will result in zero rows.
You cannot reproduce the erratic behavior with the queries as they are shown in this question since they will always return zero records. You can try it again on your end to see what you come up with, and if you do find a difference, please share it with us then.
For the future, whenever you have something like this, creating some dummy data either in the form of insert statements or in rextester or the like, you make it that much easier for someone to help you.
Best of luck.
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.
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
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.