When I have have a statement like the one below, is there a way to select all values from tableA including those that don't connect to tableB (with b.value being NULL or empty)? Thanks for any help.
SELECT DISTINCT a.value, b.value
FROM tableA a
LEFT JOIN tableB b ON ....
The behavior you described is exactly what a left join would do. There's nothing extra you need to do (except for completing the join condition you left out, of course).
Related
I have two tables:
I am looking for the results like mentioned in the last.
I tried union (only similar col can be merged), left join, right join i am getting repeated fields in Null areas what can be other options where i can get null without column repeating
A full join would get all results from both tables.
select
A.ID,
A.ColA,
A.ColB,
B.ColC,
B.ColD
from TableA A
full join Table B on A.ID = B.ID
Here is a good post to understand joins
You can try distinct:
select distinct * from
tableA a,
tableB b
where a.id = b.id;
It will not give any duplicate tuples.
For some reason I have a hard time grasping joins and this one should be very simple with the knowledge that I have in SQL.
Anyway, I have 2 tables. We will call them TableA and TableB. One of the columns in TableA is "ID". TableB only consists of the column "ID". I want to return all rows in TableA whose ID is present in TableB.
I know this should be very simple to figure out, but my brain doesn't want to work today.
You can do this using an EXISTS:
Select A.*
From TableA A
Where Exists
(
Select *
From TableB B
Where A.Id = B.Id
)
You can also use a JOIN if you wish, but depending on your data, you may want to couple that with a SELECT DISTINCT:
Select Distinct A.*
From TableA A
Join TableB B On A.Id = B.Id
One thing to keep in mind is that the ID of TableA is not necessarily related to the ID of TableB.
this should work
SELECT B.ID
FROM TableA A
JOIN TableB B
ON (A.ID=B.ID)
WHERE A.ID=B.ID
You can also use IN operator like this:
Select *
From TableA
Where ID in
(
Select distinct ID
From TableB
)
I'm only about a day into using PostgreSQL and have what is pry a quite simple question. I want to create a left outer join to search for mismatch values. I'd like from the join create a subquery which selects the null values and then updates are based on this selection. What I have for the left outer join looks like this:
SELECT TableA.field1, TableA.field2, TableB.field3
FROM
TableA
LEFT OUTER JOIN TableB ON TableA.field1 = TableB.field1
This works for the join. However, I can't manage to figure out how to then use a subquery to return only rows where TableB.field1 is null.
You don't need a subquery:
SELECT TableA.field1, TableA.field2, TableB.field1
FROM TableA
LEFT OUTER JOIN TableB ON TableA.field1 = TableB.field1
where tableB.field1 IS NULL;
Not sure what you mean with "and then updates a based on this selection" though
Here is how you ...
Select rows which are not present in other table
I suppose you want to update rows based on this selection. Use the FROM clause to join more (derived) tables in an UPDATE:
UPDATE some_table t
SET (field1, field2, field3)
= (a.field1, a.field2, a.field3)
FROM (
SELECT a.a_id, a.field1, a.field2, a.field3
FROM tablea a
LEFT JOIN tableb b USING (field1)
WHERE b.field1 IS NULL
) a
WHERE t.a_id = a.a_id; -- join on the connecting column
Using the short syntax variant to update multiple columns.
I have a table, let's call it "a" that is used in a left join in a view that involves a lot of tables. However, I only want to return rows of "a" if they also join with another table "b". So the existing code looks like
SELECT ....
FROM main ...
...
LEFT JOIN a ON (main.col2 = a.col2)
but it's returning too many rows, specifically ones where a doesn't have a match in b. I tried
SELECT ...
FROM main ...
...
LEFT JOIN (
SELECT a.col1, a.col2
FROM a
JOIN b ON (a.col3 = b.col3)) ON (a.col2 = main.col2)
which gives me the correct results but unfortunately "EXPLAIN PLAN" tells that doing it this way ends up forcing a full table scan of both a and b, which is making things quite slow. One of my co-workers suggested another LEFT JOIN on b, but that doesn't work because it gives me the b row when it's present, but doesn't stop returning the rows from a that don't have a match in b.
Is there any way to put the main.col2 condition in the sub-SELECT, which would get rid of the full table scans? Or some other way to do what I want?
SELECT ...
FROM ....
LEFT JOIN ( a INNER JOIN b ON .... ) ON ....
add a where (main.col2 = a.col2)
just do a join instead of a left join.
What if you created a view that gets you the "a" to "b" join, then do your left joins to that view?
Select ...
From Main
Left Join a on main.col2 = a.col2
where a.col3 in (select b.col3 from b) or a.col3 is null
you may also need to do some indexing on a.col3 and b.col3
First define your query between table "a" and "b" to make sure it is returning the rows you want:
Select
a.field1,
a.field2,
b.field3
from
table_a a
JOIN table_b b
on (b.someid = a.someid)
then put that in as a sub-query of your main query:
select
m.field1,
m.field2,
m.field3,
a.field1 as a_field1,
b.field1 as b_field1
from
Table_main m
LEFT OUTER JOIN
(
Select
a.field1,
a.field2,
b.field3
from
table_a a
JOIN table_b b
on (b.someid = a.someid)
) sq
on (sq.field1 = m.field1)
that should do it.
Ahh, missed the performance problem note - what I usually end up doing is putting the query from the view in a stored procedure, so I can generate the sub-queries to temp tables and put indexes on them. Suprisingly faster than you would expect. :-)
I have researched and haven't found a way to run INTERSECT and MINUS operations in MS Access. Does any way exist
INTERSECT is an inner join. MINUS is an outer join, where you choose only the records that don't exist in the other table.
INTERSECT
select distinct
a.*
from
a
inner join b on a.id = b.id
MINUS
select distinct
a.*
from
a
left outer join b on a.id = b.id
where
b.id is null
If you edit your original question and post some sample data then an example can be given.
EDIT: Forgot to add in the distinct to the queries.
INTERSECT is NOT an INNER JOIN. They're different. An INNER JOIN will give you duplicate rows in cases where INTERSECT WILL not. You can get equivalent results by:
SELECT DISTINCT a.*
FROM a
INNER JOIN b
on a.PK = b.PK
Note that PK must be the primary key column or columns. If there is no PK on the table (BAD!), you must write it like so:
SELECT DISTINCT a.*
FROM a
INNER JOIN b
ON a.Col1 = b.Col1
AND a.Col2 = b.Col2
AND a.Col3 = b.Col3 ...
With MINUS, you can do the same thing, but with a LEFT JOIN, and a WHERE condition checking for null on one of table b's non-nullable columns (preferably the primary key).
SELECT DISTINCT a.*
FROM a
LEFT JOIN b
on a.PK = b.PK
WHERE b.PK IS NULL
That should do it.
They're done through JOINs. The old fashioned way :)
For INTERSECT, you can use an INNER JOIN. Pretty straightforward. Just need to use a GROUP BY or DISTINCT if you have don't have a pure one-to-one relationship going on. Otherwise, as others had mentioned, you can get more results than you'd expect.
For MINUS, you can use a LEFT JOIN and use the WHERE to limit it so you're only getting back rows from your main table that don't have a match with the LEFT JOINed table.
Easy peasy.
Unfortunately MINUS is not supported in MS Access - one workaround would be to create three queries, one with the full dataset, one that pulls the rows you want to filter out, and a third that left joins the two tables and only pulls records that only exist in your full dataset.
Same thing goes for INTERSECT, except you would be doing it via an inner join and only returning records that exist in both.
No MINUS in Access, but you can use a subquery.
SELECT DISTINCT a.*
FROM a
WHERE a.PK NOT IN (SELECT DISTINCT b.pk FROM b)
I believe this one does the MINUS
SELECT DISTINCT
a.CustomerID,
b.CustomerID
FROM
tblCustomers a
LEFT JOIN
[Copy Of tblCustomers] b
ON
a.CustomerID = b.CustomerID
WHERE
b.CustomerID IS NULL