SQL：如果一个表为空，为什么多个表中的Max（）会为所有内容返回NULL？

Question

I have a MySQL 5.1.44 Database that I'm using with PHP 5.3.1.

SELECT MAX(t1.tstamp) AS tstamp1, MAX(t2.tstamp) AS tstamp2 FROM table1 t1, table2 t2;

This works perfectly well if both tables contain content, but it returns NULL for both (!) even if only one of the tables is empty. Why does it not simply return the correct value for table1 and null for table2?

Answer 1

Use two queries - cross join with an empty table will result zero rows

Something like this should do the work

SELECT
    (SELECT MAX(tstamp) FROM table1) AS tstamp1,
    (SELECT MAX(tstamp) FROM table2) AS tstamp2;