I am trying to run a left join on 2 tables. I do not have a group by and the only where condition i have is on the second table. But, the returned rows are less than the first table. isn't the left join suppose to bring all the data from the first table?
Here is my SQL:
select *
from tbl_a A left join tbl_b B
ON
A.Cnumber=B.Cnumber
and A.CDNUmber=B.CDNumber
and abs(A.duration - B.Duration)<2
and substr(A.text,1,3)||substr(A.text,5,8)||substr(A.text,9,2)=substr(B.text,1,8)
where B.fixed = 'b580'
There are 140,000 records in table A but the result returned is less than 100,000 records. What is the problem and how can I solve it?
As soon as you put a condition in the WHERE clause that references the right table and doesn't accommodate the NULLs that will be produced when the join is unsuccessful, you've transformed it (effectively) back into an INNER JOIN.
Try:
where B.fixed = 'b580' OR B.fixed IS NULL
Or add this condition to the ON clause for the JOIN.
You should add the where clause to the join:
select *
from tbl_a A left join tbl_b B
ON
A.Cnumber=B.Cnumber
and A.CDNUmber=B.CDNumber
and abs(A.duration - B.Duration)<2
and substr(A.text,1,3)||substr(A.text,5,8)||substr(A.text,9,2)=substr(B.text,1,8)
and B.fixed = 'b580'
If you use where statemen all records where b is not existing will not returned.
Related
I need to select all data from 2 tables in an sql database.
I searched the site and dried numerous ways but no sucess.
One table has no data but the other is full of it.
If i select each one individually i get good results, but if i use for instance:
select * from relatorio cross join temp
or
select * from relatorio r,temp t
or even:
select t.*, r.* from temp t inner join relatorio r on 1=1
The join works, but none of them shows data.
Can anyone help?
Thanks in advance.
All three select statements in the questions are cross joins.
A cross joins returns data only if both tables have at least one row.
It returns a cartesian product of both tables, meaning that every row in one table will be joined to every row in the other table.
One table has no data but the other is full of it.
Since one of your tables is empty, it will return no results at all. You can think about it as multipling by 0.
Now you have two options: one is to use a full join and the other one is to use left join, in this case both will return the same results, since one table is empty:
select *
from relatorio
left join temp on <join condition> -- assuming temp is the empty table
or
select *
from relatorio
full join temp on <join condition> -- in this case, it doesn't matter what table is empty
If you want to return all matched and umatched rows, use Full Outer Join.The FULL OUTER JOIN keyword returns all rows from the left table (table1) and from the right table (table2).
The FULL OUTER JOIN keyword combines the result of both LEFT and RIGHT joins.
In SQL the FULL OUTER JOIN combines the results of both left and right outer joins and returns all (matched or unmatched) rows from the tables on both sides of the join clause.
SQL FULL OUTER JOIN Syntax:
SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name;
The SQL CROSS JOIN produces a result set which is the number of rows in the first table multiplied by the number of rows in the second table if no WHERE clause is used along with CROSS JOIN. This kind of result is called as Cartesian Product.
If WHERE clause is used with CROSS JOIN, it functions like an INNER JOIN.
An alternative way of achieving the same result is to use column names separated by commas after SELECT and mentioning the table names involved, after a FROM clause.
CROSS JOIN SYNTAX
SELECT *
FROM table1
CROSS JOIN table2;
I have read a number of posts from SO and I understand the differences between filtering in the where clause and on clause. But most of those examples are filtering on the RIGHT table (when using left join). If I have a query such as below:
select * from tableA A left join tableB B on A.ID = B.ID and A.ID = 20
The return values are not what I expected. I would have thought it first filters the left table and fetches only rows with ID = 20 and then do a left join with tableB.
Of course, this should be technically the same as doing:
select * from tableA A left join table B on A.ID = B.ID where A.ID = 20
But I thought the performance would be better if you could filter the table before doing a join. Can someone enlighten me on how this SQL is processed and help me understand this thoroughly.
A left join follows a simple rule. It keeps all the rows in the first table. The values of columns depend on the on clause. If there is no match, then the corresponding table's columns are NULL -- whether the first or second table.
So, for this query:
select *
from tableA A left join
tableB B
on A.ID = B.ID and A.ID = 20;
All the rows in A are in the result set, regardless of whether or not there is a match. When the id is not 20, then the rows and columns are still taken from A. However, the condition is false so the columns in B are NULL. This is a simple rule. It does not depend on whether the conditions are on the first table or the second table.
For this query:
select *
from tableA A left join
tableB B
on A.ID = B.ID
where A.ID = 20;
The from clause keeps all the rows in A. But then the where clause has its effect. And it filters the rows so on only id 20s are in the result set.
When using a left join:
Filter conditions on the first table go in the where clause.
Filter conditions on subsequent tables go in the on clause.
Where you have from tablea, you could put a subquery like from (select x.* from tablea X where x.value=20) TA
Then refer to TA like you did tablea previously.
Likely the query optimizer would do this for you.
Oracle should have a way to show the query plan. Put "Explain plan" before the sql statement. Look at the plan both ways and see what it does.
In your first SQL statement, A.ID=20 is not being joined to anything technically. Joins are used to connect two separate tables together, with the ON statement joining columns by associating them as keys.
WHERE statements allow the filtering of data by reducing the number of rows returned only where that value can be found under that particular column.
please help on below query
select * from processed_h where c_type not in (select convert(int,n_index) from index_m where n_index <>'0') **-- 902 rows**
select * from processed_h where c_type not in (2001,2002,2003) **-- 902 rows**
select convert(int,n_index) from index_m where n_index <>'0' **--- 2001,2002,2003**
I tried to convert the not in to LEFT JOIN as below but it is giving me 40,000 rows returned what I am doing wrong
select A.* from processed_h A LEFT JOIN index_m B on A.c_type <> convert(int,B.n_index) and B.n_index <>'0' --40,000 + rows
A LEFT JOIN returns ALL rows from the "left-hand" table regardless of whether the condition matches or not, which is why you are getting the "extra" rows.
An INNER JOIN might give you the same number of rows, but if there are multiple matches in the "right-hand" table then you'll still get more rows than you expect.
If NOT IN gives you the expected results then I'd stick with that. You probably aren;t going to see significant improvements with a join. The only reason I would change to an INNER JOIN is if I needed columns from the joined table in my output.
For the equivalent of a NOT IN using a left join, you need to link the tables as though the results in the linked table should be IN the resultset, then select only those records where the outer joined table did not return a record - like so:
select A.* from processed_h A
LEFT JOIN index_m B on A.c_type = convert(int,B.n_index) and B.n_index <>'0'
WHERE B.n_index IS NULL
However, you might get better performance using a NOT EXISTS query instead:
select A.* from processed_h A
where not exists
(select 1 from index_m B where B.n_index <>'0' and A.c_type = convert(int,B.n_index) )
I know conditions are used in table joining. But I met a specific situation and the SQL codes writes like "Table A join table B ON TRUE"
What will happen based on the "ON TRUE" condition? Is that just a total cross join without any condition selection?
Actually, the original expression is like:
Table A LEFT outer join table B on TRUE
Let's say A has m rows and B has n rows. Is there any conflict between "left outer join" and "on true"? Because it seems "on true" results a cross join.
From what I guess, the result will be m*n rows. So, it has no need to write "left outer join", just a "join" will give the same output, right?
Yes. That's the same thing as a CROSS JOIN.
In MySQL, we can omit the [optional] CROSS keyword. We can also omit the ON clause.
The condition in the ON clause is evaluated as a boolean, so we could also jave written something like ON 1=1.
UPDATE:
(The question was edited, to add another question about a LEFT [OUTER] JOIN b which is different than the original construct: a JOIN b)
The "LEFT [OUTER] JOIN" is slightly different, in that rows from the table on the left side will be returned even when there are no matching rows found in the table on the right side.
As noted, a CROSS JOIN between tables a (containing m rows) and table b containing n rows, absent any other predicates, will produce a resultset of m x n rows.
The LEFT [OUTER] JOIN will produce a different resultset in the special case where table b contains 0 rows.
CREATE TABLE a (i INT);
CREATE TABLE b (i INT);
INSERT INTO a VALUES (1),(2),(3);
SELECT a.i, b.i FROM a LEFT JOIN b ON TRUE ;
Note that the LEFT JOIN will returns rows from table a (a total of m rows) even when table b contains 0 rows.
A cross join produces a cartesian product between the two tables, returning all possible combinations of all rows. It has no on clause because you're just joining everything to everything.
Cross join does not combine the rows, if you have 100 rows in each table with 1 to 1 match, you get 10.000 results, Innerjoin will only return 100 rows in the same situation.
These 2 examples will return the same result:
Cross join
select * from table1 cross join table2 where table1.id = table2.fk_id
Inner join
select * from table1 join table2 on table1.id = table2.fk_id
Use the last method
The join syntax's general form:
SELECT *
FROM table_a
JOIN table_b ON condition
The condition is used to tell the database how to match rows from table_a to table_b, and would usually look like table_a.some_id = table_b.some_id.
If you just specify true, you will match every row from table_a with every row of table_b, so if table_a contains n rows and table_b contains m rows the result would have m*n rows.
Most(?) modern databases have a cleaner syntax for this, though:
SELECT *
FROM table_a
CROSS JOIN table_b
The difference between the pure cross join and left join (where the condition is forced to be always true, as when using ON TRUE) is that the result set for the left join will also have rows where the left table's rows appear next to a bunch of NULLs where the right table's columns would have been.
I have a very basic LEFT OUTER JOIN to return all results from the left table and some additional information from a much bigger table. The left table contains 4935 records yet when I LEFT OUTER JOIN it to an additional table the record count is significantly larger.
As far as I'm aware it is absolute gospel that a LEFT OUTER JOIN will return all records from the left table with matched records from the right table and null values for any rows which cannot be matched, as such it's my understanding that it should be impossible to return more rows than exist in the left table, but it's happening all the same!
SQL Query follows:
SELECT SUSP.Susp_Visits.SuspReason, SUSP.Susp_Visits.SiteID
FROM SUSP.Susp_Visits LEFT OUTER JOIN
DATA.Dim_Member ON SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum
Perhaps I have made a mistake in the syntax or my understanding of LEFT OUTER JOIN is incomplete, hopefully someone can explain how this could be occurring?
The LEFT OUTER JOIN will return all records from the LEFT table joined with the RIGHT table where possible.
If there are matches though, it will still return all rows that match, therefore, one row in LEFT that matches two rows in RIGHT will return as two ROWS, just like an INNER JOIN.
EDIT:
In response to your edit, I've just had a further look at your query and it looks like you are only returning data from the LEFT table. Therefore, if you only want data from the LEFT table, and you only want one row returned for each row in the LEFT table, then you have no need to perform a JOIN at all and can just do a SELECT directly from the LEFT table.
Table1 Table2
_______ _________
1 2
2 2
3 5
4 6
SELECT Table1.Id,
Table2.Id
FROM Table1
LEFT OUTER JOIN Table2 ON Table1.Id=Table2.Id
Results:
1,null
2,2
2,2
3,null
4,null
It isn't impossible. The number of records in the left table is the minimum number of records it will return. If the right table has two records that match to one record in the left table, it will return two records.
In response to your postscript, that depends on what you would like.
You are getting (possible) multiple rows for each row in your left table because there are multiple matches for the join condition. If you want your total results to have the same number of rows as there is in the left part of the query you need to make sure your join conditions cause a 1-to-1 match.
Alternatively, depending on what you actually want you can use aggregate functions (if for example you just want a string from the right part you could generate a column that is a comma delimited string of the right side results for that left row.
If you are only looking at 1 or 2 columns from the outer join you might consider using a scalar subquery since you will be guaranteed 1 result.
Each record from the left table will be returned as many times as there are matching records on the right table -- at least 1, but could easily be more than 1.
Could it be a one to many relationship between the left and right tables?
LEFT OUTER JOIN just like INNER JOIN (normal join) will return as many results for each row in left table as many matches it finds in the right table. Hence you can have a lot of results - up to N x M, where N is number of rows in left table and M is number of rows in right table.
It's the minimum number of results is always guaranteed in LEFT OUTER JOIN to be at least N.
If you need just any one row from the right side
SELECT SuspReason, SiteID FROM(
SELECT SUSP.Susp_Visits.SuspReason, SUSP.Susp_Visits.SiteID, ROW_NUMBER()
OVER(PARTITION BY SUSP.Susp_Visits.SiteID) AS rn
FROM SUSP.Susp_Visits
LEFT OUTER JOIN DATA.Dim_Member ON SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum
) AS t
WHERE rn=1
or just
SELECT SUSP.Susp_Visits.SuspReason, SUSP.Susp_Visits.SiteID
FROM SUSP.Susp_Visits WHERE EXISTS(
SELECT DATA.Dim_Member WHERE SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum
)
Pay attention if you have a where clause on the "right side' table of a query containing a left outer join...
In case you have no record on the right side satisfying the where clause, then the corresponding record of the 'left side' table will not appear in the result of your query....
It seems as though there are multiple rows in the DATA.Dim_Member table per SUSP.Susp_Visits row.
if multiple (x) rows in Dim_Member are associated with a single row in Susp_Visits, there will be x rows in the resul set.
Since the left table contains 4935 records, I suspect you want your results to return 4935 records. Try this:
create table table1
(siteID int,
SuspReason int)
create table table2
(siteID int,
SuspReason int)
insert into table1(siteID, SuspReason) values
(1, 678),
(1, 186),
(1, 723)
insert into table2(siteID, SuspReason) values
(1, 678),
(1, 965)
select distinct t1.siteID, t1.SuspReason
from table1 t1 left join table2 t2 on t1.siteID = t2.siteID and t1.SuspReason = t2.SuspReason
union
select distinct t2.siteID, t2.SuspReason
from table1 t1 right join table2 t2 on t1.siteID = t2.siteID and t1.SuspReason = t2.SuspReason
The only way your query would return more number of rows than the left table ( which is SUSP.Susp_Visits in your case), is that the condition (SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum) is matching multiple rows in the right table, which is DATA.Dim_Member. So, there are multiple rows in the DATA.Dim_Member where identical values are present for DATA.Dim_Member.MembershipNum. You can verify this by executing the below query:
select DATA.Dim_Member.MembershipNum, count(DATA.Dim_Member.MembershipNum) from DATA.Dim_Member group by DATA.Dim_Member.MembershipNum
Simply, LEFT OUTER JOIN is the Cartesian product within each join key, along with the unmatched rows of the left table
(i.e. for each key_x that has N records in table_L and M records in table_R the result will have N*M records if M>0, or N records if M=0)