Étrange résultat lors du calcul avec des dates mysql

Question

Ce calcul select cast('2016-06-01 00:00:00' as datetime)-cast('2016-05-31 00:00:00' as datetime) a pour résultat 70 000 000 dans mySQL.

Je suis conscient que je ne devrais pas utiliser cette construction, mais je suis curieux de savoir ce que le 70000000 est?

Answer 1

Premièrement, ce n'est pas ainsi que les dates ou les dates-heures sont soustraites. On utilise datediff et timediff pour cela. Je dis juste. Donc, voir MySQL Date and Time Functions.

Mais regardant

select cast('2016-06-01 00:00:00' as datetime)-cast('2016-05-31 00:00:00' as datetime); 

select cast('2016-06-01 00:00:00' as datetime)-cast('2016-05-30 00:00:00' as datetime); 

select cast('2016-06-01 00:00:00' as datetime)-cast('2016-06-02 00:00:00' as datetime); 

-1000000

select cast('2016-06-01 00:00:00' as datetime)-cast('123' as char(3)); 

20160600999877;

On peut voir un motif d'eux étant traités comme bigints. Alors

select 20160601000000 - 20160531000000; 

select 20160601000000 - 20160530000000; 

Alors que cela semble être la raison.

Answer 2

Comme @Drew indique, lors de l'exécution de soustraction sont traités comme BIGINT:

$ mysql --column-type-info 

mysql> SELECT 
    -> `der`.`DATE_1`, 
    -> `der`.`DATE_2`, 
    -> `der`.`DATE_1` - `der`.`DATE_2` DIFF 
    -> FROM (SELECT CAST('2016-06-01 00:00:00' AS DATETIME) `DATE_1`, 
    ->    CAST('2016-05-31 00:00:00' AS DATETIME) `DATE_2`) `der`; 
Field 1: `DATE_1` 
Catalog: `def` 
Database: `` 
Table:  `der` 
Org_table: `` 
Type:  DATETIME 
Collation: binary (63) 
Length:  19 
Max_length: 19 
Decimals: 0 
Flags:  BINARY 

Field 2: `DATE_2` 
Catalog: `def` 
Database: `` 
Table:  `der` 
Org_table: `` 
Type:  DATETIME 
Collation: binary (63) 
Length:  19 
Max_length: 19 
Decimals: 0 
Flags:  BINARY 

Field 3: `DIFF` 
Catalog: `def` 
Database: `` 
Table:  `` 
Org_table: `` 
Type:  LONGLONG 
Collation: binary (63) 
Length:  16 
Max_length: 8 
Decimals: 0 
Flags:  BINARY NUM 

-- 20160601000000  20160531000000 
+---------------------+---------------------+----------+ 
| DATE_1    | DATE_2    | DIFF  | 
+---------------------+---------------------+----------+ 
| 2016-06-01 00:00:00 | 2016-05-31 00:00:00 | 70000000 | 
+---------------------+---------------------+----------+ 
1 row in set (0,00 sec)