the case is that I need to select a field distinct from table1 (no duplicates) and use the result as a key to select from another table2. And I need this to be in one query. Is this possible?!
table1: hID, hName, hLocation
table2: hID, hFrom, hTo, hRate, hRoomType, hMeals
I want to correct version of this query:
SELECT
*
FROM
table1
JOIN (
DISTINCT
hID
FROM
table2
WHERE
hRoomType = Double Room
ON table1.hID = table2.hID)
expected result: all hotels that offer Double Room thanks much –
thanks for help!
Your question is quite vague and confusing. Is this what you are looking for:
SELECT hID, name, location
FROM table2
INNER JOIN table1
ON table1.hID = table2.hID
GROUP BY table2.hID;
Here is a skeleton to achieve this:
SELECT
* -- Don't forget to list the requested fields instead of using `*`!
FROM (
-- This is the distinct list from table1
SELECT DISTINCT
id
FROM
table1 T1
) DT1
INNER JOIN table2 T2
ON T1.id = T2.reference_to_t1_id
Another solution if you don't want to retrieve any columns from table1:
SELECT
* -- Don't forget to list the requested fields instead of using `*`!
FROM
table2 T2
WHERE
-- Sais that get all record from table2 where this condition matches
-- at least one record
EXISTS (
SELECT 1 FROM table1 T1 WHERE T1.id = T2.reference_to_t1_id
)
For your tables and question
SELECT
hID, hName, hLocation
FROM
table1 T1
WHERE
EXISTS (
SELECT 1 FROM
table2 T2
WHERE
T1.hID = T2.hID
AND T.hRoomType = 'Double' -- Assuming that this is the definition of double rooms
)
Related
I need some help with a specific problem about subqueries in MySQL. This is a shortversion of the problem I have:
I have table 1 and 2 where table 1 have a column called "SupplyID" which corresponds with table 2's "ID". I need to get a list with stuff from one single supplier.
I tried with this but didn't work:
select name
from item
where SupplyID exists (
select *
from SupplyID
where SupplyID = ID
);
Assuming your tables are named table1 and table2 you
You could use a inner join
select distinct t1.name
from ybale as t1
inner join table2 as t2 on t1.ID = t2.SupplyID
Try this:
select name from item i where exists (select * from table2 t where i.SupplyID = t.ID);
My answer is more like to what scaisEdge answered here but I strongly recommend to do this with LEFT JOIN. and If you want to select just one ID, for example ID=10
SELECT T1.name
FROM item AS T1
LEFT JOIN SupplyID AS T2 ON T2.SupplyID=T1.ID
WHERE T1.ID = 10
I have 2 tables:
table1 (id,usedcode)
table2 (codeid,uniquecode)
I want to be able to check if a certain value exists in uniquecode of Table2, but is not already used in Table1
Try using left join as below:
SELECT t2.*
FROM table2 t2 LEFT JOIN table1 t1
ON t2.uniquecode = t1.usedcode
WHERE t1.usedcode IS null
SELECT uniquecode FROM Table2
WHERE NOT EXISTS(
SELECT * FROM Table1 WHERE usedcode = uniquecode
)
In English the query is saying, "Select all unique codes from table 2 that don't exist in table 1 as a usedcode".
I need to update a table consisting of million rows
there are two tables table1 and table2
SELECT ID
FROM (
select ID from table1 where<condition>
) as result1
INNER JOIN table2 ON result1.field=table2.field
GROUPBY table2.field
HAVING <condtion>
and on this #resultset1 ID, I have to update table 1
UPDATE table1
SET x=true
where ID EXISTS IN (#resultset1)
there are millions of rows in both table. how do i do it?
And Can anyone say whats wrong with this, i am trying some alternative over join
UPDATE table1 t1
SET x=true
WHERE <condition> AND EXISTS(
SELECT* FROM (
SELECT *
FROM table2 t2
WHERE t2.field = t1.field
) AS result
WHERE<condition on resultset field>
);
Output the results of the first query to a file.
Convert the output to update statements, one update per id (using sed or whatever)
Split the statements into separate files, maybe a 1000 per file (using split or whatever)
Run each file as an sql script with a pause (eg 10 seconds) between each execution (to allow logs to update etc and spread the load), using a simple runner script the loops over the files
Why can't you simply do like below. Though I don't really see a need of group by and having provided if this is your actual query.
UPDATE table1
SET x=true
where ID IN (
SELECT ID
FROM (
select ID from table1 where<condition>
) as result1
INNER JOIN table2 ON result1.field=table2.field
GROUPBY table2.field
HAVING <condtion>
)
Another good option would be JOIN both result set and do the UPDATE like
UPDATE table1 t1
JOIN
(
SELECT ID
FROM (
select ID from table1 where<condition>
) as result1
INNER JOIN table2 ON result1.field=table2.field
GROUPBY table2.field
HAVING <condtion>
) X ON t1.ID = X.ID
SET t1.x=true
EDIT:
You can as well store the result of fist query in a temporary table (As already suggested by a_horse_with_no_name) and then do the UPDATE using a JOIN with the temporary table. Somethig like below
create temporary table idtemp(ID INT);
insert into idtemp
SELECT ID
FROM (
select ID from table1 where<condition>
) result1
INNER JOIN table2 ON result1.field=table2.field
GROUPBY table2.field
HAVING <condtion>
Finally do the update like
UPDATE table1 t1
JOIN idtemp it ON t1.ID = it.ID
SET t1.x=true
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.
I have 2 tables for which I need to run a query on
Table1 has 2 fields: l_id, and name
Table2 also has 2 fields: l_id, and b_id
I need to run a query to get the "name" and "l_id" for all the entries in table1 that do not have an entry in table2 for a given b_id.
Hope this makes some sense
select t1.*
from Table1 t1
left outer join Table2 t2 on t1.l_id = t2.l_id
and t2.b_id = #SomeValue
where t2.l_id is null
You can use an outer join, but I find a sub-query is a little more straightforward. In your case selecting everything from table1 that does not have an id in table2. Reads better...
SELECT * FROM table1 WHERE l_id NOT IN (SELECT l_id FROM table2);