UNION & UNION ALL

Introducere în SQL Server

John MacKintosh

Instructor

SELECT 
  album_id, 
  title, 
  artist_id 
FROM album 
WHERE artist_id IN (1, 3)

+------------+-------------------------+------------|
| album_id   | title                   | artist_id  |
|------------+-------------------------|------------|
| 1          | For Those About To Rock | 1          |
| 4          | Let There Be Rock       | 1          |
| 5          | Big Ones                | 3          |
+------------+----------+---------------------------+

SELECT 
  album_id, 
  title, 
  artist_id 
FROM album 
WHERE artist_id IN (1, 4, 5)

+------------+-------------------------+------------|
| album_id   | title                   | artist_id  |
|------------+-------------------------|------------|
| 1          | For Those About To Rock | 1          |
| 4          | Let There Be Rock       | 1          |
| 6          | Jagged Little Pill      | 4          |
| 7          | Facelift                | 5          |
+------------+-------------------------+------------+

Combinarea rezultatelor

SELECT 
  album_id, 
  title, 
  artist_id 
FROM album 
WHERE artist_id IN (1, 3)

UNION

SELECT 
  album_id, 
  title, 
  artist_id 
FROM album 
WHERE artist_id IN (1, 4, 5);

+------------+-------------------------+------------|
| album_id   | title                   | artist_id  |
|------------+-------------------------|------------|
| 1          | For Those About To Rock | 1          |
| 4          | Let There Be Rock       | 1          |
| 5          | Big Ones                | 3          |
| 6          | Jagged Little Pill      | 4          |
| 7          | Facelift                | 5          |
+------------+-------------------------+------------+

Rândurile duplicate sunt excluse

UNION ALL

SELECT 
  album_id, 
  title, 
  artist_id 
FROM album 
WHERE artist_id IN (1, 3) 
UNION ALL 
SELECT 
  album_id, 
  title, 
  artist_id 
FROM album 
WHERE artist_id IN (1, 4, 5);

+------------+-------------------------+------------|
| album_id   | title                   | artist_id  |
|------------+-------------------------|------------|
| 1          | For Those About To Rock | 1          |
| 4          | Let There Be Rock       | 1          |
| 5          | Big Ones                | 3          |
| 1          | For Those About To Rock | 1          |
| 4          | Let There Be Rock       | 1          |
| 6          | Jagged Little Pill      | 4          |
| 7          | Facelift                | 5          |
+------------+-------------------------+------------+

Include rândurile duplicate

Crearea de noi nume de coloane pentru rezultate finale

SELECT 
  album_id AS ALBUM_ID, 
  title AS ALBUM_TITLE, 
  artist_id AS ARTIST_ID 
FROM album 
WHERE artist_id IN(1, 3) 
UNION ALL 
SELECT 
  album_id AS ALBUM_ID, 
  title AS ALBUM_TITLE, 
  artist_id AS ARTIST_ID 
FROM album 
WHERE artist_id IN(1, 4, 5)

+------------+-------------------------+------------|
| ALBUM_ID   | ALBUM_TITLE             | ARTIST_ID  |
|------------+-------------------------|------------|
| 1          | For Those About To Rock | 1          |
| 4          | Let There Be Rock       | 1          |
| 5          | Big Ones                | 3          |
| 1          | For Those About To Rock | 1          |
| 4          | Let There Be Rock       | 1          |
| 6          | Jagged Little Pill      | 4          |
| 7          | Facelift                | 5          |
+------------+-------------------------+------------+

Rezumat

UNION sau UNION ALL: combină interogări din aceeași tabelă sau din tabele diferite

La combinarea datelor din tabele diferite:

Selectați același număr de coloane, în aceeași ordine
Coloanele trebuie să aibă aceleași tipuri de date

Dacă tabelele sursă au nume de coloane diferite

Utilizați aliasuri pentru coloane

UNION: elimină duplicatele (mai lent)

UNION ALL: include duplicatele (mai rapid)

Să exersăm!

Introducere în SQL Server