MySQL null problem continued

When group_concat and Concat are mentioned above, if the column value is null, the value after Concat is null.

Another problem is found. If Column A is null and column B is null, the sum of the number of records of column A <> B and A = B is not equal to the total number.

That is to say, if column A is null and column B is null '', MySQL will not compare them, that is, this situation is neither equal nor equal.

The solution is to use the ifnull (sqlserver is isnull) function: ifnull (,''). Convert null to null before comparison with B.

In addition, when the select count (idc_position.id) from ** statement is run, if the field ID of some rows in the table is null, when the select count (ID) Statement returns the number of record rows, the rows whose ID is null are not included, and these rows are automatically killed.

Another trap ......