I want to join 3 tables:
Table A: Date, CountOfHonda(Integer)
Table B: Date, CountOfToyota(Integer)
Table C: Date, CountOfMazda(Integer)
I want the result table as: Date, CountOfHonda,CountOfToyota,CountOfMazda
All the dates might not be present in all 3 tables. I want to, in a sense, combine the tables without eliminating or duplicating any date value in the final table. If a Date is missing in a table, the value of the corresponding count column can be null/0. Thanks!
use full outer join simulation.
basically you do this:
SELECT * FROM tableA
LEFT JOIN tableB ON tableA.Date = tableB.Date
UNION
SELECT * FROM tableA
RIGHT JOIN tableB ON tableA.Date = tableB.Date
UNION
SELECT * FROM tableA
LEFT JOIN tableC ON tableA.Date = tableC.Date
UNION
SELECT * FROM tableA
RIGHT JOIN tableC ON tableA.Date = tableC.Date
don't worry about the dates which are not present in other tables. that's what the "outer" join is for. it would still keep those rows.
Edit: forgot mysql didn't have a full outer join. but this should take care of your query.
What you want is a tally (or lookup) table, for instance, DateLookupTable.
This table would have a single column, Date, with values from 01/01/1900 - 12/31/2099 (or whatever range is appropriate.
Then you do..
select d.Date, a.CountOfHonda, b.CountOfToyota, c.CountOfMazda
from DateLookupTable d
left join tableA a on a.Date = d.Date
left join tableB b on b.Date = d.Date
left join tableC c on c.Date = d.Date
where d.Date between (startDate) and (endDate)
Tally/lookup tables are invaluable in many situations.
Related
Imagine the following scenario:
There are 3 tables A, B and C.
Table A has no knowledge of either table B and table C.
Table B has a foreign key to table A.
Table C has foreign key to table B.
In table B as well as in table C there can be multiple items sharing the same foreign key value.
As you can see, the items from C are indirectly referenced to A through B.
What I want is to get all entries from A that are referenced in C but without any information from B or C in my result tables and without duplicates.
Is this even possible?
I have tried this like so but have no idea if it is correct:
select tableA.*
from tableA,
(select distinct tableB.AId as Aid
from tableB left join tableC on tableC.BId = tableB.id
group by tableB.id)
as temp
where tableA.id = temp.Aid
I am not sure if I understand it correctly, but you can try this one:
SELECT DISTINCT `A`.`id`, `A`.`value1`, `A`.`value2` FROM `A`
INNER JOIN `B` ON `B`.`id-a` = `A`.`id`
INNER JOIN `C` ON `C`.`id-b` = `B`.`id`
It returns all values from table A if there is a key on Table C which is linked to Table B with corresponding foreign key on table A
An alternative approach to Masoud's good response would be to use an exists though a correlated subquery.
The below subquery joins B to C in a correlated fashion (notice the B.IDA to A.ID and A is outside the subquery).
If we assume good database design, then A will not have duplicate records, thus we can omit a distinct here since we are not joining A to the other tables. Instead we are simply checking for the existence of an "A" record in the B table which must have a record in the C table due to the inner join. This has two advantages for performance
It doesn't have to join all the records together which would then
necessitate a distinct; thus you don't have the performance hit on
the distinct.
It can early escape. once a key value of A is found in the
subquery (B to C join) , it can stop looking and thus don't have to join all of B to all of A.
We select "1" in the subquery as we don't care what we select as the value will not be used anywhere. We're just using the coloration of A to (B JOIN C) to determine what in A to display.
SELECT A.*
FROM A
WHERE EXISTS( SELECT 1
FROM C
INNER JOIN B
on C.IDB = B.ID)
AND B.IDA = A.ID)
Taking what you tried and reviewing it:
select tableA.*
from tableA,
(select distinct tableB.AId as Aid
from tableB left join tableC on tableC.BId = tableB.id
group by tableB.id)
as temp
where tableA.id = temp.Aid
Starting with the "FROM"
You have tableA, (subquery) temp. This is a CROSS JOIN meaning all records from A will be joined to ALL records of (B JOIN C) so if you have 1000 records in A and 1000 records in the temp result then you'd be telling the database engine to generate 1000*1000 records in your result set; which then gets filtered to only include records matching in temp and A. The engine may be smart enough to avoid the cross join and optimize the query, but I find it confusing to maintain. So I would rewrite as
SELECT tableA.*
FROM tableA
INNER JOIN (SELECT distinct tableB.AId as Aid
FROM tableB left join tableC on tableC.BId = tableB.id
GROUP BY tableB.id) as temp
ON tableA.id = temp.Aid
Looking at the subquery (temp)
We don't need a group by as we are not aggregating. The distinct does bring us down to 1 record but at a cost to execution time.
So I would re-write as this:
SELECT tableA.*
FROM tableA
INNER JOIN (SELECT distinct tableB.AId as Aid
FROM tableB
LEFT JOIN tableC
on tableC.BId = tableB.id) as temp
ON tableA.id = temp.Aid
Then looking at the whole, if we change the outer query join to temp and make it an exists... using coloration we don't have the performance hit of the join, nor the distinct. and I'd switch the left join to an inner as we only want records in C and B so we'd have null in B if we left it as a "LEFT JOIN" which serve no purpose for us.
This gets me to the answer I initially provided.
SELECT tableA.*
FROM tableA
WHERE EXISTS (SELECT 1
FROM tableB
INNER JOIN tableC
on tableC.BId = tableB.id
AND tableB.AID = A.ID) as temp
I have 3 tables A, B, C
Schema of all 3 tables is same as mentioned below:
1st A table:
cpid ,name, place
2nd B table:
connectorid,dob
3rd C table:
ccpid cconnectorid
Now both tables A and B have many records.
Now some of the records in A and B are with same id.
Now I want to merge the records from A and B into Table C.
Merge logic is as follows
1)If records with cpid = connectorid ,insert into table c.
2)C Table ccpid is the foreignkey for A table cpid and cconnectorid is the foreignkey B table connectorid.
3)Using select query.
You can use select insert with a n inner join
insert into table_c
select a.cpid, b.connectorid, a.place
from table_b as b
inner join table_a as a on a.id = b.id
You can try this solution for your query:
INSERT INTO `C`(`ccpid`, `cconnectorid`, `ccity`)
SELECT ta.`cpid`, ta.`cconnectorid`, tb.`place`
FROM `A` as ta
INNER JOIN `B` tb ON ta.`cpid` = tb.`cconnectorid`
You just need join data from both tables? This is simple JOIN function.
SELECT *
FROM Table_A
INNER JOIN Table_B
ON Table_A.cpid =Table_B.connectorid;
You can insert this select to your Table_C.
Here is INNER JOIN, but I think you should take a look to JOINs, here are examples and you can read more about other JOINs.
INNER JOIN: Returns all rows when there is at least one match in BOTH
tables LEFT JOIN: Return all rows from the left table, and the matched
rows from the right table RIGHT JOIN: Return all rows from the right
table, and the matched rows from the left table FULL JOIN: Return all
rows when there is a match in ONE of the tables
use following query replace with your table names
INSERT INTO CTABLE(ccpid,cconnectorid,ccity)
(SELECT A.cpid ,B.connectorid, A.place FROM
TABLEA A INNER JOIN TABLEB B ON A.cpid = B.connectorid)
I have two tables lets say
Table A
columns id , name address
Table B
columns id , age, import_date
The Table B id is a reference key of Table A.
Now I want to return results from A & B but if the record is not in B I still want to see the record so for this I use left outer join
Select * from A a left join B b
on a.id = b.id
Now even I don't have record in B I still get the record.
Table B may contain duplicate ids but unique import_date.
Now I want to results in a way that if there is duplicate id in table B then I want to get the records only where import_date is as of today.
I still want to get the records for ids which are not there but if the ID is there in table B then I want to apply above condition.
I hope someone can help me with this.
Sample data
Table A
01|John|London
02|Matt|Glasgow
03|Rodger|Paris
Table B
02|22|31-AUG-2015
02|21|30-AUG-2015
02|23|29-AUG-2015
The query will return
01|John|London|null|null|null
02|Matt|Glasgow|22|31-Aug-2015
03|Rodger|Paris|null|null
You almost got the solution. Just add one more condition like below
Select a.id,a.name,a.address,b.age,b.import_date
from tablea a left join tableb b
on a.id=b.id and b.import_date=trunc(sysdate)
order by a.id;---This line optional
Check the DEMO HERE
SELECT *
FROM Table_A t1 LEFT OUTER JOIN Table_B t2 ON t1.id=t2.id UNION
SELECT *
FROM Table_A t1 LEFT OUTER JOIN Table_B t2 ON t1.id=t2.id
GROUP BY t2.import_date
HAVING t2.import_date=CURDATE();
I need a little bit of help in creating this query. I'm joining TableA and TableB and getting a value out of it; then joining TableA and TableC and getting a value out if it. Finally I am substracting both values.
I'm not sure how to write this in a single query using a lot of JOIN or if I just do 2 subqueries and then substract them.
So far I have something like:
SELECT SUM(A.quantity) From TableA JOIN Table B WHERE ...
then
SELECT SUM(A.quantity) From TableA JOIN Table C WHERE ...
Given the chance that maybe TableA and TableB have no result, but TableA and TableC does, or viceversa, or maybe both have or maybe both won't, I can't just JOIN TableA and TableB and TableC
You can do this with a cross join:
select coalesce(s1.q1, 0) - coalesce(s2.q2, 0)
from (SELECT SUM(A.quantity) as q1 From TableA JOIN Table B WHERE ...) s1 cross join
(SELECT SUM(A.quantity) as q2 From TableA JOIN Table C WHERE ...) s2;
If one of the result sets returns NULL, the coalesce() treats the value as 0.
I want to get the SUM(column_a) from two different tables, and get their difference. I am using MySQL.
Table A's sum = 1234
Table B's sum = 4001
I'm not sure what to put in my ON clause:
SELECT
SUM(a.column1) AS table_a_sum,
SUM(b.column1) AS table_b_sum,
SUM(a.column1) - SUM(b.column1) AS difference
FROM table_a a
JOIN table_b b
ON ??????
A join without condition is a cross join. A cross join repeats each row for the left hand table for each row in the right hand table:
FROM table_a a
CROSS JOIN table_b b
Note that in MySQL, cross join / join / inner join are identical. So you could write:
FROM table_a a
JOIN table_b b
As long as you omit the on clause, this will work as a cross join.
If you'd like to sum two columns from two tables, a cross join would not work because it repeats rows. You'd get highly inflated numbers. For sums, a better approach uses subqueries, per #sgeddes answer.
Here's one option using subqueries -- there are several ways to do this:
SELECT
table_a_sum,
table_b_sum,
table_a_sum - table_b_sum AS difference
FROM
(SELECT SUM(column1) table_a_sum FROM table_a) a,
(SELECT SUM(column1) table_b_sum FROM table_b) b
You want to summarize first and then do the calculations:
select a.suma, b.sumb, a.suma - b.sumb
from (select sum(a.column1) as suma from tablea) a cross join
(select sum(b.column1) as sumb from tableb) b
Doing the cross join between the tables will generate a cartesian product that will mess up your sums.