Good! If you now compare the results of INNER JOIN with the content of the Equipment table (expand the Database tab on the right), you'll notice that not all pieces of equipment are present in the result set. For example, a lovely kettle with the ID 11 is not there. Do you know why?
INNER JOIN (or JOIN) only shows those rows from the two tables where there is a match between the columns. In other words, you can only see those pieces of equipment that have a room assigned, and vice versa. A piece of equipment with no room is not shown in the result. Take a look at the table:
| Equipment | Room | |||||
|---|---|---|---|---|---|---|
| Id | Name | RoomId | Id | RoomNumber | Beds | Floor |
| 1 | kettle | 4 | 4 | 104 | 3 | 1 |
| 2 | fridge | 5 | 5 | 201 | 1 | 2 |
| 3 | tv | 8 | 8 | 204 | 3 | 2 |
| 5 | kettle | 7 | 7 | 203 | 3 | 2 |
| 6 | radio | 7 | 7 | 203 | 3 | 2 |
| 7 | computer | 7 | 7 | 203 | 3 | 2 |
| 8 | toaster | 1 | 1 | 101 | 2 | 1 |
| 9 | toaster | 1 | 1 | 101 | 2 | 1 |
| 12 | kettle | 2 | 2 | 102 | 2 | 1 |
| 13 | tv | 3 | 3 | 103 | 3 | 1 |
| 14 | microwave | 9 | 9 | 205 | 4 | 2 |
| 15 | computer | 10 | 10 | 301 | 4 | 3 |
| 4 | tv | NULL | ||||
| 10 | microwave | NULL | ||||
| 11 | kettle | NULL | ||||
Blue rows are the result of the INNER JOIN. Equipment with NULL value in the RoomID column (pink rows) is not shown in the INNER JOIN result.
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