How to remove null values from Mysql

In Mysql, the field null has two different meanings: 1. The field value is null; 2. The field value is a null string to remove the null value: SQL code create table B select column 1, column 2, IFNULL (column 3, 'I am null') as column 3, IFNULL (column 4,-999) as column 4... from table A; Description: in Table A, if column 3 is NULL, it is replaced with the string 'I am null'. If Column 4 is NULL, it is replaced with the value-999. (The return value of the IFNULL function. If the column is not empty, the original value is returned. The type of the value is consistent according to the field type; otherwise, the value you defined is returned .) Remove null strings (using the update statement): SQL code update table A set field 1 = 'value you want to replace 'where Field 1 = '';