[solved] list Size() = 1 but all elements are null

background ：

Use mybatis Aggregate function in （max()、avg()） Inquire about SQL sentence , Load the returned results List aggregate

The following method is used to determine the null value ：

if (CollectionUtils.isNotEmpty(list)) {

return xxx;
}

The result is , No matter what Whether there are values in the database , Return is not empty .

further debug Find out , list.size() = 1, but List The median value is empty , Show All elements are null

Question why ：

Used MySQL Aggregate function in , These are summation functions SUM()、 Find the average function AVG()、 Maximum function MAX()、 Minimum function MIN() And counting functions COUNT, Which leads to size = 1, But when the result is empty , The specific reason is unknown , Subsequent updates ..

terms of settlement ：

Use the following method to judge the blank

if (CollectionUtils.isNotEmpty(list) && list.get(0) != null) {

return xxx;
}