As far as I understand in a left outer join between two tables (say a & b) all the rows of the table on the left side of the join are retrieved regardless of the values in the rows on the right table. Then why do we need an 'ON' clause specifying a condition, something like this:
select * from a LEFT OUTER JOIN b on a.some_column1 = b.some_column2;
Why is there a need for the statement "a.some_column1 = b.some_column2".
A left join would return all the rows from table a, and for each row the matching row in table b, if it exists - if it doesn't, nulls would be returned instead of b's columns. The on clause defines how this matching is done.
An on clause is required since you are "joining", and you need to tell which columns you want to join by. Otherwise you would use traditional from without any where condition to all possible row combinations. But you wanted a join, right?
Yeah, that's pretty much it is.
As far as I understand in a left outer join between two tables (say a & b) all the rows of the table on the left side of the join are retrieved regardless of the values in the rows on the right table.
That is correct in the sense that it says something about what left join on returns, but it isn't a definition of what it returns. left join on returns inner join on rows plus (union all) unmatched left table rows extended by nulls.
inner join on returns the rows of cross join that satisfy the on condition--which could be any condition on the columns. cross join returns every combination of a row from the left table & a row from the right table.
What do you expect outer join without on to mean? In standard SQL outer & inner join have to have an on. inner join on a true condition is the same as cross join. Which has no unmatched left table rows. So if you want outer join with no on to mean outer join on a true condition then, since there are no unmatched rows in the inner join on that condition, the result is also just cross join. (MySQL allows inner join to be used without an on, but it just means cross join.)
Related
Does inner join of two empty tables lead to cross join?
According to Wikipedia: Cross join
CROSS JOIN returns the Cartesian product of rows from tables in the
join.
In other words, it will produce rows which combine each row from the
first table with each row from the second table.[1]
Taking the above into consideration, the cross join of two empty tables will produce an empty resultset.
The join of two empty tables also will produce an empty result. Because according to the definition the join is a speciall case of cross join
The result of the join can be defined as the outcome of first taking
the Cartesian product (or Cross join) of all rows in the tables
(combining every row in table A with every row in table B) and then
returning all rows which satisfy the join predicate.
so yes - one can say that the inner join of two empty tables lead to cross join
I have two sql to join two table together:
select top 100 a.XXX
,a.YYY
,a.ZZZ
,b.GGG
,b.JJJ
from table_01 a
left join table_02 b
on a.XXX = b.GGG
and b.JJJ = "abc"
and a.YYY between '01/08/2009 13:18:00' and '12/08/2009 13:18:00'
select top 100 a.XXX
,a.YYY
,a.ZZZ
,b.GGG
,b.JJJ
from table_01 a
left join table_02 b
on a.XXX = b.GGG
where b.JJJ = "abc"
and a.YYY between '01/08/2009 13:18:00' and '12/08/2009 13:18:00'
The outcome of them is different but I don't understand the reason why.
I would be grateful if I can get some help here.
Whenever you are using LEFT JOIN, all the conditions about the content of the right table should be in the ON clause, otherwise you are effectively converting your LEFT JOIN to an INNER JOIN.
The reason for that is that when a LEFT JOIN is used, all the rows from the left table will be returned. If they are matched by the right table, the values of the matching row(s) will be returned as well, but if they are not matched with any row on the right table, then the right table will return a row of null values.
Since you can't compare anything to NULL (not even another NULL) (Read this answer to find out why), you are basically telling your database to return all rows that are matched in both tables.
However, when the condition is in the ON clause, Your database knows to treat it as a part of the join condition.
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 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.
This question already has answers here:
What is the difference between "INNER JOIN" and "OUTER JOIN"?
(28 answers)
Closed 9 years ago.
Difference between inner and outer join. i am using two table and want to fetch data from both table so which type join we should use owning of that we can solve our problem
This is the best and simplest way to understand joins:
Credits go to the writer of this article HERE
Inner join - An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.
Left outer join -
A left outer join will give all rows in A, plus any common rows in B.
Full outer join -
A full outer join will give you the union of A and B, i.e. All the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.
check this
INNER JOIN: Returns all rows when there is at least one match in BOTH tables
LEFT JOIN: Return all rows from the left table, and the matched rows from the right table
RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table
FULL JOIN: Return all rows when there is a match in ONE of the tables
Inner join matches tables on keys, but outer join matches keys just for one side.
For example when you use left outer join the query brings the whole left side table and matches the right side to the left table primary key and where there is not matched places null.