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Set operations allow queries to be combined according to set operation semantics. Set operations refer to the UNION [ALL]
, INTERSECT [ALL]
and EXCEPT [ALL]
clauses. The vanilla variants use set semantics, i.e., they eliminate duplicates, while the variants with ALL
use bag semantics.
Traditional set operations unify queries by column position, and require the to-be-combined queries to have the same number of input columns. If the columns are not of the same type, casts may be added. The result will use the column names from the first query.
DuckDB also supports UNION [ALL] BY NAME
, which joins columns by name instead of by position. UNION BY NAME
does not require the inputs to have the same number of columns. NULL
values will be added in case of missing columns.
UNION
The UNION
clause can be used to combine rows from multiple queries. The queries are required to return the same number of columns. Implicit casting to one of the returned types is performed to combine columns of different types where necessary. If this is not possible, the UNION
clause throws an error.
Vanilla UNION
(Set Semantics)
The vanilla UNION
clause follows set semantics, therefore it performs duplicate elimination, i.e., only unique rows will be included in the result.
SELECT * FROM range(2) t1(x)
UNION
SELECT * FROM range(3) t2(x);
x |
---|
2 |
1 |
0 |
UNION ALL
(Bag Semantics)
UNION ALL
returns all rows of both queries following bag semantics, i.e., without duplicate elimination.
SELECT * FROM range(2) t1(x)
UNION ALL
SELECT * FROM range(3) t2(x);
x |
---|
0 |
1 |
0 |
1 |
2 |
UNION [ALL] BY NAME
The UNION [ALL] BY NAME
clause can be used to combine rows from different tables by name, instead of by position. UNION BY NAME
does not require both queries to have the same number of columns. Any columns that are only found in one of the queries are filled with NULL
values for the other query.
Take the following tables for example:
CREATE TABLE capitals (city VARCHAR, country VARCHAR);
INSERT INTO capitals VALUES
('Amsterdam', 'NL'),
('Berlin', 'Germany');
CREATE TABLE weather (city VARCHAR, degrees INTEGER, date DATE);
INSERT INTO weather VALUES
('Amsterdam', 10, '2022-10-14'),
('Seattle', 8, '2022-10-12');
SELECT * FROM capitals
UNION BY NAME
SELECT * FROM weather;
city | country | degrees | date |
---|---|---|---|
Seattle | NULL | 8 | 2022-10-12 |
Amsterdam | NL | NULL | NULL |
Berlin | Germany | NULL | NULL |
Amsterdam | NULL | 10 | 2022-10-14 |
UNION BY NAME
follows set semantics (therefore it performs duplicate elimination), whereas UNION ALL BY NAME
follows bag semantics.
INTERSECT
The INTERSECT
clause can be used to select all rows that occur in the result of both queries.
Vanilla INTERSECT
(Set Semantics)
Vanilla INTERSECT
performs duplicate elimination, so only unique rows are returned.
SELECT * FROM range(2) t1(x)
INTERSECT
SELECT * FROM range(6) t2(x);
x |
---|
0 |
1 |
INTERSECT ALL
(Bag Semantics)
INTERSECT ALL
follows bag semantics, so duplicates are returned.
SELECT unnest([5, 5, 6, 6, 6, 6, 7, 8]) AS x
INTERSECT ALL
SELECT unnest([5, 6, 6, 7, 7, 9]);
x |
---|
5 |
6 |
6 |
7 |
EXCEPT
The EXCEPT
clause can be used to select all rows that only occur in the left query.
Vanilla EXCEPT
(Set Semantics)
Vanilla EXCEPT
follows set semantics, therefore, it performs duplicate elimination, so only unique rows are returned.
SELECT * FROM range(5) t1(x)
EXCEPT
SELECT * FROM range(2) t2(x);
x |
---|
2 |
3 |
4 |
EXCEPT ALL
(Bag Semantics)
EXCEPT ALL
uses bag semantics:
SELECT unnest([5, 5, 6, 6, 6, 6, 7, 8]) AS x
EXCEPT ALL
SELECT unnest([5, 6, 6, 7, 7, 9]);
x |
---|
5 |
8 |
6 |
6 |