问题
What is the difference between CROSS JOIN and FULL OUTER JOIN in SQL Server?
Are they the same, or not? Please explain. When would one use either of these?
回答1:
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.
A full outer join is a combination of a left outer and right outer join. It returns all rows in both tables that match the query's where clause, and in cases where the on condition can't be satisfied for those rows it puts null values in for the unpopulated fields.
This wikipedia article explains the various types of joins with examples of output given a sample set of tables.
回答2:
One thing that might not always be obvious to some is that a cross join with an empty table (or result set) results in empty table (M x N; hence M x 0 = 0)
A full outer join will always have rows unless both M and N are 0.
回答3:
I'd like to add one important aspect to other answers, which actually explained this topic to me in the best way:
If 2 joined tables contain M and N rows, then cross join will always produce (M x N) rows, but full outer join will produce from MAX(M,N) to (M + N) rows (depending on how many rows actually match "on" predicate).
EDIT:
From logical query processing perspective, CROSS JOIN does indeed always produce M x N rows. What happens with FULL OUTER JOIN is that both left and right tables are "preserved", as if both LEFT and RIGHT join happened. So rows, not satisfying ON predicate, from both left and right tables are added to the result set.
回答4:
Cross join :Cross Joins produce results that consist of every combination of rows from two or more tables. That means if table A has 3 rows and table B has 2 rows, a CROSS JOIN will result in 6 rows. There is no relationship established between the two tables – you literally just produce every possible combination.
Full outer Join : A FULL OUTER JOIN is neither "left" nor "right"— it's both! It includes all the rows from both of the tables or result sets participating in the JOIN. When no matching rows exist for rows on the "left" side of the JOIN, you see Null values from the result set on the "right." Conversely, when no matching rows exist for rows on the "right" side of the JOIN, you see Null values from the result set on the "left."
回答5:
For SQL Server, CROSS JOIN and FULL OUTER JOIN are different.
CROSS JOIN is simply Cartesian Product of two tables, irrespective of any filter criteria or any condition.
FULL OUTER JOIN gives unique result set of LEFT OUTER JOIN and RIGHT OUTER JOIN of two tables. It also needs ON clause to map two columns of tables.
Table 1 contains 10 rows and Table 2 contains 20 rows with 5 rows matching on specific columns.
Then
CROSS JOINwill return 10*20=200 rows in result set.
FULL OUTER JOINwill return 25 rows in result set.
FULL OUTER JOIN(or any other JOIN) always returns result set with less than or equal toCartesian Product number.Number of rows returned by
FULL OUTER JOINequal to (No. of Rows byLEFT OUTER JOIN) + (No. of Rows byRIGHT OUTER JOIN) - (No. of Rows byINNER JOIN).
回答6:
Cross Join: http://www.dba-oracle.com/t_garmany_9_sql_cross_join.htm
TLDR; Generates a all possible combinations between 2 tables (Carthesian product)
(Full) Outer Join: http://www.w3schools.com/Sql/sql_join_full.asp
TLDR; Returns every row in both tables and also results that have the same values (matches in CONDITION)
回答7:
Hi they are the same concepts apart from the NULL value returned.
See below:
declare @table1 table ( col1 int, col2 int )
declare @table2 table ( col1 int, col2 int )
insert into @table1 select 1, 11 union all select 2, 22
insert into @table2 select 10, 101 union all select 2, 202
select *
from @table1 t1 full outer join @table2 t2
on t1.col1 = t2.col1
/* RESULT
col1 col2 col1 col2
----------- ----------- ----------- -----------
NULL NULL 10 101
2 22 2 202
1 11 NULL NULL
(3 row(s) affected)
*/
select *
from @table1 t1 cross join @table2 t2
/* RESULT
col1 col2 col1 col2
----------- ----------- ----------- -----------
1 11 10 101
2 22 10 101
1 11 2 202
2 22 2 202
(4 row(s) affected)
*/
回答8:
Full outer join :
This join combines left outer join and right outer join. It returns row from either table when the conditions are met and returns null value when there is no match.
image : (http://www.pinaldave.com/bimg/March09UG/outer%20join.jpg)
Cross Join :
This join is a Cartesian join that does not necessitate any condition to join. The result set contains records that are multiplication of record number from both the tables.
image : (http://www.pinaldave.com/bimg/March09UG/cross%20join%20-%20half.jpg)
回答9:
Here is an example where both the FULL OUTER JOIN and CROSS JOIN return the same result set without NULL returned. Please note the 1 = 1 in the ON clause for the FULL OUTER JOIN:
declare @table1 table ( col1 int, col2 int )
declare @table2 table ( col1 int, col2 int )
insert into @table1 select 1, 11 union all select 2, 22
insert into @table2 select 10, 101 union all select 2, 202
select *
from @table1 t1 full outer join @table2 t2
on 1 = 1
(2 row(s) affected) (2 row(s) affected) col1 col2 col1 col2 ----------- ----------- ----------- ----------- 1 11 10 101 2 22 10 101 1 11 2 202 2 22 2 202
select *
from @table1 t1 cross join @table2 t2
col1 col2 col1 col2 ----------- ----------- ----------- ----------- 1 11 10 101 2 22 10 101 1 11 2 202 2 22 2 202 (4 row(s) affected)
回答10:
x cross join y is x full join y on 1=1.
PS
x cross join yis rows that start with the column names & values of a row ofx& finish with the column names & values of a row ofy.x , yisx cross join ybut with lower precedence than the keywordjoins.x inner join y on cis the rows ofx cross join ythat makectrue.
(That is x cross join y where c.)
x left join y on cisx inner join y on cunion allrows that start like a row ofxthat did not makectrue with any row iny& finish with the columns ofyall with valuenull.x right join y on cisx inner join y on cunion allrows that start with the columns ofxall with valuenull& finish like a row ofythat did not makectrue with any row inx.
(If you ignore column order then x left join y on c is y right join x on c.)
x full join y on cisx inner join y on cunion allrowsx left join y on cadds beyondx inner join y on cunion allrowsx right join y on cadds beyondx inner join y on c.
(That is x left join y on c union all rows x right join y on c adds beyond x inner join y on c.
And it is x right join y on c union all rows x left join y on c adds beyond x inner join y on c.)
(So x cross join y is
x inner join y on 1=1
& x left join y on 1=1
& x right join y on 1=1
& x full join y on 1=1.)
Memorize the bullets.
Always know what inner join you want as part of an outer join.
回答11:
SQL FULL OUTER JOIN
The FULL OUTER JOIN returns all rows from the left table (table1) and from the right table (table2) irrespective of the match.
The FULL OUTER JOIN keyword combines the result of both LEFT OUTER JOIN and RIGHT OUTER JOIN
- SQL full outer join is also known as FULL JOIN
Reference : http://datasciencemadesimple.com/sql-full-outer-join/
SQL CROSS JOIN
In SQL CROSS JOIN Each Row of first table is mapped with the each and every row of second table.
Number of rows produced by a result set of CROSS JOIN operation is equal to number of rows in the first table multiplied by the number of rows in the second table.
CROSS JOIN is also known as Cartesian product / Cartesian join
Number of rows in table A is m, Number of rows in table B is n and resultant table will have m*n rows
Reference:http://datasciencemadesimple.com/sql-cross-join/
来源：https://stackoverflow.com/questions/3228871/sql-server-what-is-the-difference-between-cross-join-and-full-outer-join