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Wd提问: (select 198766*667891 from DUAL)

Ai回答: The SQL query:

```
(SELECT 198766 * 667891 FROM DUAL)
```

is a valid SQL statement, but its syntax and usage depend on the database system being used.

🔍 What is `DUAL`?

- `DUAL` is a dummy table in some database systems, such as Oracle.
- It is used when you want to perform a query that doesn't reference any actual table — for example, to compute an expression or get the current date/time.

In Oracle, this is a common pattern:
```sql
SELECT 198766 * 667891 FROM DUAL;
```

This will return the result of the multiplication:
132,750,682,106

🚫 In Other Databases (e.g., MySQL, PostgreSQL)

- In MySQL or PostgreSQL, there is no `DUAL` table.
- You can simply write:
```sql
SELECT 198766 * 667891;
```

If you try to use `FROM DUAL` in these databases, you'll get an error like:
```
ERROR: relation "dual" does not exist
```

✅ Summary

| Database | Valid? | Notes |
||--|-|
| Oracle | ✅ Yes | Uses `DUAL` for scalar queries |
| MySQL | ❌ No | No `DUAL` table; just `SELECT ...` |
| PostgreSQL| ❌ No | Same as MySQL |

🔢 Result of the Calculation

As before:

```
198,766 × 667,891 = 132,750,682,106
```

Would you like to know how this query might be used in real applications or how to avoid SQL injection?