I have a $member_id, there are 5 tables which this member details are stored in those tables in multiple rows.
For getting out this user data from those tables, I can use JOIN and UNION:
//Using JOIN:
SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id LEFT JOIN table3 ON table2.id=table3.id AND table1.member_id = '$member_id'
//USING UNION
(SELECT a FROM t1 WHERE member_id = '$member_id')
UNION
(SELECT a FROM t2 WHERE member_id = '$member_id')
ORDER BY a LIMIT 10;
Which one is preffered? which one has the better performance? what's the true way while you want to get some information from many tables which are related with foreign keys?
If you need all columns in same row you should use join. For a row by column you should use union. This is the criteria. They are no secret.
Remember to use left joins if this column don't appear in all tables:
SELECT table0.id as a0, table1.id as a1, table2.id as a2, ...
FROM
(select '$member_id' as id from dual ) table0
LEFT outer join table1 ON table1.id=table0.id
Left outer JOIN table2 ON table1.id=table2.id
LEFT outer JOIN table3 ON table2.id=table3.id
Also, in union, you can tag each row:
//USING UNION
(SELECT 't1' as t, a FROM t1 WHERE member_id = '$member_id')
UNION ALL
(SELECT 't2' as t, a FROM t2 WHERE member_id = '$member_id')
ORDER BY a LIMIT 10;
If you have the same structure and want to remove duplicate rows you can use UNION otherwise it's better to use JOIN statement
Related
I want to get value from table1 and join all matching value from table2. The table1 has to be limited to 2 rows, but expecting output should own all matching values for those two ids.
How can I achieve this?
You would use a subquery:
select t1.*, t2.*
from (select t1.*
from table1 t1
limit 10
) t1 left join
table2 t2
on t1.id = t2.table1_id;
Note: This returns two arbitrary rows. Normally, you would have an order by to better specify the rows. And use order by rand() for random rows.
if you want all the value in join for only two row of table 1 you can use a subqiuery with limit 2
select b.id, a.value, b.value2, b.table1_ID
from (
select * from table1
limit 2
) a
inner join table2 on aid = b.table1_ID
I've two tables, table1 contains 22780 rows. Now I left join table1 with table2 (which doesn't contain any duplicates) and I get 23588 rows.
SELECT * FROM Table1
left join Tabelle6 ON CAST(Table1.Customer AS Int) = table2.Customer
Why do I get more rows now? I only need every row from table1 once.
Edit: found my issue, table 2 does contain duplicates. But is there any way to join every row only once and ignore any further matches?
As the comment suggests, the easiest way to handle this would probably be to do SELECT DISTINCT to remove duplicates from your result set:
SELECT DISTINCT
t1.col1,
t1.col2,
t1.Customer,
...
FROM Table1 t1
LEFT JOIN Table2 t2
ON CAST(t1.Customer AS Int) = t2.Customer
But there is another option here. We could also join to a subquery which removes duplicate customers. This would ensure that no record from the first table gets duplicated from matching to more than one record in the second table.
SELECT *
FROM Table1 t1
LEFT JOIN
(
SELECT DISTINCT Customer
FROM Table2
) t2
ON CAST(t1.Customer AS Int) = t2.Customer
I have 2 tables t1 and t2. Each have a customer ID column. What I am looking for is to join the 2 columns and SUBTRACT the duplicates.
My EG:
Table1 and Table2 with the IDs for each
I have tried a union query. The result I am left with is ID = 1,2,3,4,5,6,7,8,9,10. Where, what I'm after is subtracting 1-5 from Table2 and the result = 6,7,8,9,10.
I hope that makes sense and that someone is able to help. Sorry if this is a bit too simple compared to what you're all used to.
In SQL Server you can use the EXCEPT operator:
select ID
from Table2
except
select ID
from Table1
Mysql does not support it though. Using a an in clause or a left join would work in both servers:
--Using In clause
SELECT ID
FROM Table2
WHERE ID NOT IN
(
SELECT ID
FROM Table1
);
--Using join
SELECT Table2.ID
FROM Table2
left join Table1
on Table2.ID = Table1.ID
where Table1.ID is null
Use left outer join
select * from t1 left outer join t2 on t1.customerid = t2.customerid
Query 1:
SELECT sum(total_revenue_usd)
FROM table1 c
WHERE c.irt1_search_campaign_id IN (
SELECT assign_id
FROM table2 ga
LEFT JOIN table3 d
ON d.campaign_id = ga.assign_id
)
Query 2:
SELECT sum(total_revenue_usd)
FROM table1 c
LEFT JOIN table2 ga
ON c.irt1_search_campaign_id = ga.assign_id
LEFT JOIN table3 d
ON d.campaign_id = ga.assign_id
Query 1 gives me the correct result where as I need it in the second style without using 'in'. However Query 2 doesn't give the same result.
How can I change the first query without using 'in' ?
The reason being is that the small query is part of a much larger query, there are other conditions that won't work with 'in'
You could try something along the lines of
SELECT sum(total_revenue_usd)
FROM table1 c
JOIN
(
SELECT DISTINCT ga.assign_id
FROM table2 ga
JOIN table3 d
ON d.campaign_id = ga.assign_id
) x
ON c.irt1_search_campaign_id = x.assign_id
The queries do very different things:
The first query sums the total_revenue_usd from table1 where irt1_search_campaign_id exists in table2 as assign_id. (The outer join to table3 is absolutely unnecessary, by the way, because it doesn't change wether a table2.assign_id exists or not.) As you look for existence in table2, you can of course replace IN with EXISTS.
The second query gets you combinations of table1, table2 and table3. So, in case there are two records in table2 for an entry in table1 and three records in table3 for each of the two table2 records, you will get six records for the one table1 record. Thus you sum its total_revenue_usd sixfold. This is not what you want. Don't join table1 with the other tables.
EDIT: Here is the query using an exists clause. As mentioned, outer joining table3 doesn't alter the results.
Select sum(total_revenue_usd)
from table1 c
where exists
(
select *
from table2 ga
-- left join table3 d on d.campaign_id = ga.assign_id
where ga.assign_id = c.irt1_search_campaign_id
);
Is it possible to get 1 result where I require data from 3 tables.
First table: I will need to grab all the fields (1 row found by a primary key)
Second table: I will need to grab the field 'username' (connected to first table by 'master_id')
Third table: I will need to grab the latest added row with the associated master_id key (table has 'date', 'master_id', 'previous_name').
select top 1 first.*, second.username, third.*
from first
inner join second on first.id = second.master_id
inner join third on first.id = third.master_id
order by
third.date desc
As always there are dozens of ways to skin a cat, I'm not sure if this is optimized as the subquery methods, but it should work.
You can join the three tables together. Then, you can use a "filter" join to keep only the latest Table3 row:
select *
from Table1 t1
join Table2 t2
on t2.master_id = t1.master_id
join Table3 t3
on t3.master_id = t1.master_id
join (
select master_id
, max(date) as max_date
from Table3
group by
master_id
) as filter
on t3.master_id = filter.master_id
and t3.date = filter.max_date
You'll need a correlated subquery for that third table.
SELECT t1.*, username, date, previous_name
FROM FirstTable t1
INNER JOIN SecondTable t2 ON t1.master_id=t2.master_id
INNER JOIN
(SELECT master_id, date, previous_name
FROM ThirdTable AS t3_1
WHERE date = (
SELECT MAX(date)
FROM ThirdTable AS t3_2
WHERE t3_2.master_id=t3_1.master_id)) q1 ON q1.master_id=t1.master_id;
NOTE: Untested.