MySQL "invalid use of null value" solution

1. Problem description

Because to change ”information” In the table ” Number ” Column is not empty , Use database query statements “alter table information modify ‘ Number ’ varchar(255) not null” You can change other columns , But it's just an error in the number column , The error message is Invalid use of NULL value.

Is very strange , Other columns can be queried by using this command , Why can't you check only the number column .

2. The reason for the error

The reason for this error may be the existing data ‘ Number ’ As a null, And not null The settings of conflict .（ The reason found ）.

I think it's because when I first created a new data table , The table structure of this column has been set , When making subsequent changes, it will conflict with the previous settings （ As for why it can't be changed here , I don't quite understand ）, Because I set the structure attribute of the numbered column when creating a new table .

Since it cannot be set to non empty , So create a new column and set not null, You can succeed .

3. terms of settlement

1） Add new column , Set the structure properties of the column .

alter table information add column ‘yyy’ varchar(255) not null first; - Add a new column yyy To the first column .

2） Copy the contents of the wrong column into the new column and delete the wrong column

update information set yyy=’ Number ’; — take ’ Number ’ All values of the column are copied to yyy Column .

alter table information drop ‘ Number ’; — Delete the wrong column .

3） Modify the new column name to the wrong column name

alter table information change ‘yyy’ ‘ Number ’ varchar(255) not null;

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