SQL: Multi-table SELECT

Multi-table `SELECT` is query like
```SQL
SELECT * FROM foo, bar;
```

### Example
If `foo` table is:
| a | b |
|---|---|
| 1 | 2 |
| 2 | 1 |

and `bar` is
| c | d |
|---|---|
| 3 | 4 |
| 4 | 3 |

Query result:
| a | b | c | d |
|---|---|---|---|
| 1 | 2 | 3 | 4 |
| 1 | 2 | 4 | 3 |
| 2 | 1 | 3 | 4 |
| 2 | 1 | 4 | 3 |