I am trying to fetch records from 2 tables mapped by an id where on the second table there may be a row that is missing.
I have a column called name on the second table which contains a string value. The value I need to extract is 'subscriptions' but this does not always exist in the table. There is the possibility to have different values within this column which I do not want to extract.
Is it possible to check to see if the value exists and if it doesn't output null to all the fields.
So far I have this which returns all the records
select COUNT(*)
from PUser a, PAttribute b
where exists (select null
from PAttribute c
where c.name = 'subscriptions' or c.name is null)
and a.id = b.userid;
Hope that explains it.
EDIT
PUser table
id
other columns
PAttribute table
userid mapped to PUser.id
name
Now a userid can have multiple rows each with a different value in name eg, 'subscriptions', 'source', 'etc' 'etc'
I want to fetch all users who have the value 'subscriptions' in the name column or if the row doesnt exist with the value 'subscriptions' as they may not have any.
If they don't have this row the output should be null.
EDIT 2:
Worked this out and I needed
select COUNT(*),(select b.stringValue from PAttribute b where b.userid = a.id and b.name = 'subscriptions') from PUser a order by a.id desc;
Your example is using implicit joins, which are inner joins. This means that a result will only be returned if a row exists in both tables. Instead, you need to use a left join. Change your query to this:
select COUNT(*)
from PUser a LEFT JOIN PAttribute b ON a.id = b.userID
where exists (select null
from PAttribute c
where c.name = 'subscriptions' or c.name is null);
Or (not exactly sure what your desired behavior is), this might work for you:
SELECT count(*)
FROM PUser a LEFT JOIN PAttribute b ON a.id = b.userID
WHERE b.name = 'subscriptions' OR b.name IS NULL;
If you want to exclude rows that do not contain 'subscriptions', you could use the JOIN ON form and in order to keep rows from PUser even there is no matching row from PAttribute with name set to 'subrciptions', and thus obtaining null fields, exploit OUTER JOIN.
select COUNT(*)
from PUser a OUTER JOIN PAttribute b ON ( a.id = b.userid AND b.name = 'subscriptions' )
;
This is a little bit different from your query: EXISTS is less perfomant and, moreover, the SELECT in the EXISTS does search for a row in PAttribute with name equal to null, that is quite different from handling missing rows.
Related
I have 2 mysql tables a and b. I need to show all records in a that are not in b using a common column 'ID'. Normally this is pretty straight forward but my problem here is this: I need to put a where clause on table b because I'm not interested in all unmatched records just the ones that meet the table 2 where clause criterion:
SELECT a.ID, a.Description
FROM a
LEFT JOIN b ON a.ID = b.ID
WHERE a.Inactive = 0
AND b.Room = '101'
AND b.ID Is Null
This returns nothing. However if I remove the AND b.Room = '101' part, it displays the expected results. But I need that condition because I need only unmatched records specific to a 'room' and not all unmatched records
Move the conditions involving table b from WHERE to ON clause:
SELECT a.ID, a.Description
FROM a
LEFT JOIN b ON a.ID = b.ID AND b.Room = '101'
WHERE a.Inactive = 0
AND b.ID Is Null
It'll find rows where a does not have a match in b (id matches and room number = 101).
I have two tables. A and B
SELECT projectid, statusid from A;
Results as below:
SELECT statusid, status from B
Results as below:
Now how can I have the result as below to replace all stutusid with the value from table B.
You could use JOIN:
SELECT A.projectid, B.status from A JOIN b ON A.statusid = b.statusid;
EDIT:
I want to have all those status codes replaced by "its meaning" which is from another table B. So "join" wont do this job but only showing me matched results from both A and B table
I don't understand the requirements. This is exactly what join is doing, showing corresponding value from second table. If for example you don't have the specific code you should use LEFT OUTER JOIN
SELECT A.projectid, COALESCE(B.status, 'Unknown') AS status
from A
LEFT JOIN b ON A.statusid = b.statusid;
Maybe you are searching for correlated subquery(but it will be literally the same as JOIN) if there is no duplicates on table B
SELECT A.projectid,
(SELECT B.status FROM B WHERE A.statusid = b.statusid) AS status
FROM A
-- if it returns error query returned more than one row then b.statusid is not unique
SELECT A.projectid,
(SELECT B.status FROM B WHERE A.statusid = b.statusid ORDER BY ... LIMIT 1) AS status
FROM A
I've this table (a)
And this table (b)
Now I have to get all records from A which are not present in B (a.id not present as b.idDomanda) and where B.idUser is not 1. So In this case, it should return only id 2 from a, but it returns 1 and 2.
This is my Query
SELECT a.* FROM a LEFT JOIN b ON a.id=b.idDomanda WHERE ( b.idUser <> 1 OR b.idUser IS NULL ) GROUP BY a.id
You want to move the condition on b to the on clause:
SELECT a.*
FROM a LEFT JOIN
b
ON a.id = b.idDomanda and b.idUser <> 1
WHERE b.idUser IS NULL
GROUP BY a.id;
The group by suggests that you might want to use not exists instead:
select a.*
from a
where not exists (select 1
from b
where a.id = b.idDomanda and b.idUser <> 1
);
There should be no results given your data set.
All records from A which are not present in B (a.id not present as b.idDomanda)
Given the test data set all of A is in fact IN b.idDomanda... even when filtering out userId = 1.
but as the previous person pointed out that is the query to check.
I've 3 tables say A,B,C.
Table A has userid column.
Table B has caid column.
Table C has lisid and image columns.
one userid can have one or several caids.
one caid can have one or several lisids.
how do I select a userid which has maximum number of rows with image column as not null (in some lisids image column is blank and in some it has some value).
can someone please help.
Presumably, the ids are spread among the tables in a reasonable fashion. If so, the following should do this:
select b.userid, count(*)
from TableB b join
TableC c
on b.caid = c.caid
where c.image is not null
group by b.userid
order by count(*) desc
limit 1
The question in the comments is how you connect TableA to TableB and TableB to TableC. The reasonable approach is to have the userid in TableB and the caid in TableC.
Getting all the rows with the max requires a bit more work. Essentially, you have to join in the above query to get the list
select s.*
from (select b.userid, count(*) as cnt
from TableB b join
TableC c
on b.caid = c.caid
) s
(select count(*) as maxcnt
from TableB b join
TableC c
on b.caid = c.caid
group by b.userid
order by count(*) desc
limit 1
) smax
on s.cnt = smax.cnt
Other databses have a set of functions called window functions/ranking functions that make this sort of query much simpler. Alas, MySQL does not offer these.
i have a table called users contained
(Table A)
Users
user_id
username
user_type[1,2]
(Table B) if user_type=1
user_id
full_name
(Table C) if user_type=2
user_id
full_name
i want to get single record set by executing single query, is that possible in PHP mysql.
Try this:
SELECT table_a.*, COALESCE(table_b.full_name,table_c.full_name) AS full_name
FROM table_a
LEFT OUTER JOIN table_b ON table_b.user_id = table_a.user_id
LEFT OUTER JOIN table_c ON table_c.user_id = table_a.user_id WHERE 1;
It uses the LEFT OUTER JOIN, which means that it joins it to table_b on the given condition. However, for each row in table_a, whether it finds a matching row in table_b or not, it will return the table_a row. If it does not find a matching row, the table_b columns are just NULL. Same thing with table_c.
Then, we just select all the table_a columns. However, we now have two full_name columns, one from table_b and one from table_c. We use COALESCE to merge them.
COALESCE returns the first non-NULL value.
Since we know that there is either a matching row in table_b or a matching row in table_c, it is not a problem. However, it would be a problem if somehow you allows a matching row to be found in both table_b and table_c.
The risk can be mitigated by adding additional ON clause conditions to get:
SELECT table_a.*, COALESCE(table_b.full_name,table_c.full_name) AS full_name
FROM table_a
LEFT OUTER JOIN table_b
ON table_b.user_id = table_a.user_id AND table_a.user_type = 1
LEFT OUTER JOIN table_c
ON table_c.user_id = table_a.user_id AND table_a.user_type = 2
WHERE 1;
Still, you will need to make sure only 1 row is present for each user in table_b and table_c.
Instead of COALESCE you can optionally use CASE like:
SELECT table_a.*, CASE user_type WHEN 1
THEN table_b.full_name
ELSE table_c.full_name END AS full_name
...
or use an IF function like:
SELECT table_a.*, IF(user_type=1,table_b.full_name,table_c.full_name) AS full_name
...
You can UNION both tables and later on JOIN it with tableA
SELECT a.User_ID,
a.`username`,
b.full_name,
a.user_type
FROM tableA a
(
SELECT user_ID, full_name
FROM tableB
UNION
SELECT user_ID, full_name
FROM tableC
) b ON a.User_ID = b.User_ID
-- WHERE a.user_type = 1 -- add extra condition here