If I have a table with the following atributes:
A: id, race, key1
B: key1, driving_id
C: driving_id, fines
why would it be possible for us to have the following queries:
select A.id, A.race, B.key1, B.driving_id, C.fines
from A
left join B on A.key1=B.key1
left join C on B.driving_id= C.driving_id
even though there are no common keys for A and C in the last line of the SQL query?
The query that you have written is parsed as:
select A.id, A.race, B.key1, B.driving_id, C.fines
from (A left join
B
on A.key1 = B.key1
) left join
C
on B.driving_id = C.driving_id;
That is, C is -- logically -- being joined to the result of A and B. Any keys from those tables would be valid.
Although your original query is the preferable way to write it, you could also write:
select ab.id, ab.race, ab.key1, ab.driving_id, C.fines
from (select . . . -- whatever columns you need
from A left join
B
on A.key1 = B.key1
) ab left join
C
on ab.driving_id = C.driving_id;
The three versions are all equivalent, but the last one may help you better understand what is going on with joins between multiple tables.
Without seeing sample data from the three tables, we might not know for sure in the query makes any sense or would even run. Assuming it does run, then there should be nothing wrong with the join logic. For example, it is perfectly possible for table B to have a key key1 which relates to the A table, while at the same time having another key driving_id which relates to the C table. Note that either of these keys (but not both) could be a primary key in the B table, and if not then each key would be a foreign key.
The LEFT JOIN keyword returns all records from the left table (tableA), and the matched records from the right table (tableB). Furthermore, Similarly it returns all records from the result of first set, and the matched records from the right table (tableC). The result is NULL from the right side, if there is no match.
So A & C have a link through table B.
Related
I have to join (required to use join) these 2 tables table and table B.
I have table A
CoverID
4!12569
3!18175
1!478931
And Table B
ID
Accountid
4
12569
3
18175
1
478931
Please advise how can I join the table using concat
From TableA A
Left join TableB B
On A.CoverId = (concat(B.ID, ‘!’, B.AccountId))
You need to make sure the data types match. So you’ll have to play around with whether or not you need to cast the data to match within the concatenation besides that it should work.
Questions similar to this one about using DISTINCT values in an INNER JOIN have been asked a few times, but I don't see my (simple) use case.
Problem Description:
I have two tables Table A and Table B. They can be joined via a variable ID. Each ID may appear on multiple rows in both Table A and Table B.
I would like to INNER JOIN Table A and Table B on the distinct values of ID which appear in Table B and select all rows of Table A with a Table A.ID which appears matching some condition in Table B.
What I want:
I want to make sure I get only one copy of each row of Table A with a Table A.ID matching a Table B.ID which satisfies [some condition].
What I would like to do:
SELECT * FROM TABLE A
INNER JOIN (
SELECT DISTINCT ID FROM TABLE B WHERE [some condition]
) ON TABLE A.ID=TABLE B.ID
Additionally:
As a further (really dumb) constraint, I can't say anything about the SQL standard in use, since I'm executing the SQL query through Stata's odbc load command on a database I have no information about beyond the variable names and the fact that "it does accept SQL queries," ( <- this is the extent of the information I have).
If you want all rows in a that match an id in b, then use exists:
select a.*
from a
where exists (select 1 from b where b.id = a.id);
Trying to use join just complicates matters, because it both filters and generates duplicates.
I have two tables, Table A and Table B. Each table have 4 fields, the name of the fields are the same for both. Both tables are extracted from other tables, and each record acts as a primary key.
I want to write a query in MS Access 2010 that gets the data unique to Table B and not shared with Table A. I am using the following image as a reference, and it looks like I need to do a Right Join.
Hello. There is something not right with my SQL, I've tested it and I am getting the incorrect result. Below is the closest I've gotten:
SELECT DISTINCT TableB.*
FROM TableB RIGHT JOIN TableA ON (TableB.Field1 = TableA.Field1) AND (TableB.Field2 = TableA.Field2) AND (TableB.Field3 = TableA.Field3) AND (TableB.Field4 = TableA.Field4)
WHERE (((TableA.Field1) Is Null));
I think it would be clearer for you to use not exists:
select tableb.*
from tableb
where not exists (select 1
from tablea
where (TableB.Field1 = TableA.Field1) AND (TableB.Field2 = TableA.Field2) AND (TableB.Field3 = TableA.Field3) AND (TableB.Field4 = TableA.Field4)
);
Your use of RIGHT JOIN is incorrect. As phrased, you want a LEFT JOIN. That is, you want to keep all rows in the first table (the "left" table in the JOIN) regardless of whether or not a match exists in the second table. However, the NOT EXISTS does the same thing and the logic is a bit clearer.
You want to have right join if tablea is in your select statement, but as you have
SELECT DISTINCT TableB.*
you may want to have a left join instead. My suggestion would be changing your code from right to left join.
TableB acts like table A from venn diagrams above.
I have been working on this issue since 2 days now.
I have two tables created by using SQL Select statements
SELECT (
) Target
INNER JOIN
SELECT (
) Source
ON Join condition 1
AND Join condition 2
AND Join condition 3
AND Join condition 4
AND Join condition 5
The target table has count value of 10,000 records.
The source table has count value of 10,000 records.
but when I do an inner join between the two tables on the 5 join conditions
I get 9573 records.
I am basically trying to find a one to one match between source and target table. I feel every field from target matches every field in source.
Questions:
Why does my inner join give less records even if there are same value of records in both tables?
If it is expected, how can I make sure I get the exact 10,000 records after the join condition?
1) An INNER JOIN only outputs the rows from the JOINING of two tables where their joining columns match. So in your case, Join Condition1 may not exist in rows in both tables and therefore some rows are filtering out.
2) As the other poster mentioned a left join is one way. You need to look which table source or target you want to use as your master i.e. start from and return all those rows. You then left join the remaining table based on your conditions to add all the columns where you join conditions match.
It's probably better if you give us the tables you are working on and the query\results you are trying to achieve.
There's some really good articles about the different joins out there. But it looks like you'd be interested in left joins. So if it exists in Target, but not in Source, it will not drop the record.
So, it would be:
SELECT(...) Target
LEFT OUTER JOIN
SELECT(...) Source
ON cond1 and cond2 and cond3 and cond4 and cond5
Give that a shot and let me know how it goes!
Sometime you need to rely on logical analysis rather than feelings. Use this query to find the fields that do not match and then work out your next steps
SELECT
Target.Col1,Source.Col1,
Target.Col2,Source.Col2,
Target.Col3,Source.Col3
FROM
(
) Target
FULL OUTER JOIN
(
) Source
ON Target.Col1=Source.Col1
AND Target.Col2=Source.Col2
AND Target.Col3=Source.Col3
WHERE (
Target.Col1 IS NULL
OR Source.Col1 IS NULL
OR Target.Col2 IS NULL
OR Source.Col2 IS NULL
OR Target.Col3 IS NULL
OR Source.Col3 IS NULL
)
I want to delete records from a table using inner joins on more than two tables. Say if I have tables A,B,C,D with A's pk shared in all other mentioned tables. Then how to write a delete query to delete records from table D using inner joins on table B and A since the conditions are fetched from these two tables. I need this query from DB2 perspective. I am not using IN clause or EXISTS because of their limitations.
From your description, I take the schema as:
A(pk_A, col1, col2, ...)
B(pk_B, fk_A, col1, col2, ..., foreign key fk_A references A(pk_A))
C(pk_c, fk_A, col1, col2, ..., foreign key fk_A references A(pk_A))
D(pk_d, fk_A, col1, col2, ..., foreign key fk_A references A(pk_A))
As you say DB2 will allow only 1000 rows to be deleted if IN clause is used. I don't know about DB2, but Oracle allows only 1000 manual values inside the IN clause. There is not such limit on subquery results in Oracle at least. EXISTS should not be a problem as any database, including Oracle and DB2 checks only for existence of rows, be it one or a million.
There are three scenarios on deleting data from table D:
You want to delete data from table D in which fk_A (naturally) refers to a record in table A using column A.pk_A:
DELETE FROM d
WHERE EXISTS (
SELECT 1
FROM a
WHERE a.pk_A = d.fk_A
);
You want to delete data from table D in which fk_A refers to a record in table A, and that record in table A is also referred to by column B.fk_A. We do not want to delete the data from D that is in A but not in B. We can write:
DELETE FROM d
WHERE EXISTS (
SELECT 1
FROM a
INNER JOIN b ON a.pk_A = b.fk_A
WHERE a.pk_A = d.fk_A
);
The third scenario is when we have to delete data in table D that refers to a record in table A, and that record in A is also referred by columns B.fk_A and table C.fk_A. We want to delete only that data from table D which is common in all the four tables - A, B, C and D. We can write:
DELETE FROM d
WHERE EXISTS (
SELECT 1
FROM a
INNER JOIN b ON a.pk_A = b.fk_A
INNER JOIN c ON a.pk_A = c.fk_A
WHERE a.pk_A = d.fk_A
);
Depending upon your requirement you can incorporate one of these queries.
Note that "=" operator would return an error if the subquery retrieves more than one line. Also, I don't know if DB2 supports ANY or ALL keywords, hence I used a simple but powerful EXISTS keyword which performs faster than IN, ANY and ALL.
Also, you can observe here that the subqueries inside the EXISTS clause use "SELECT 1", not "SELECT a.pk" or some other column. This is because EXISTS, in any database, looks for only existence of rows, not for any particular values inside the columns.
Based on 'Using SQL to delete rows from a table using INNER JOIN to another table'
The key is that you specify the name of the table to be deleted from
as the SELECT. So, the JOIN and WHERE do the selection and limiting,
while the DELETE does the deleting. You're not limited to just one
table, though. If you have a many-to-many relationship (for instance,
Magazines and Subscribers, joined by a Subscription) and you're
removing a Subscriber, you need to remove any potential records from
the join model as well.
DELETE subscribers
FROM subscribers INNER JOIN subscriptions
ON subscribers.id = subscriptions.subscriber_id
INNER JOIN magazines
ON subscriptions.magazine_id = magazines.id
WHERE subscribers.name='Wes';
delete from D
where fk = (select d.fk from D d,A a,B b where a.pk = b.fk and b.fk = d.fk )
this should work