SQL и NULL
Интерактивное демо «SQL и NULL» — раздел Энциклопедия · Данные и разметка.
SQL и NULL
Трёхзначная логика — NULL = NULL → UNKNOWN
= UNKNOWN
| a | op | b | result |
|---|---|---|---|
| NULL | = | NULL | UNKNOWN |
| NULL | = | 5 | UNKNOWN |
| NULL | = | 0 | UNKNOWN |
| 5 | = | NULL | UNKNOWN |
| 5 | = | 5 | TRUE |
| 5 | = | 0 | FALSE |
| 0 | = | NULL | UNKNOWN |
| 0 | = | 5 | FALSE |
| 0 | = | 0 | TRUE |
| NULL | IS NULL | NULL | TRUE |
| NULL | IS NULL | 5 | TRUE |
| NULL | IS NULL | 0 | TRUE |