After set up the only constraint in MySQL can add multiple null? I see on the video said can only add a null, but how do I operation can add more?
Then there is the database having whether must be after the group by group to use?
, every brother please help answer, thank you!

CodePudding user response:

The first question, please put your table paste out look at the script, estimation is built table script has a problem, can also be set at the bottom of the problem,
The second question, must have a GROUP BY,