如何计算所有COUNT的总和？

Question

I am new to mysql .

I have a query to get count from different tables and it works okay. But, now I want to club all 3 query COUNT into 1 like SUM (COUNT1, COUNT2, COUNT3)

select COUNT(*) as count1, 0 as count2, 0 as count3 from table1 
UNION ALL 
select 0 as count1, COUNT(*) as count2, 0 as count3 from table2 
UNION ALL 
select 0 as count1, 0 as count2, COUNT(*) as count3 from table3

Eg count1 = 10, count2 = 20 and count3 = 15

Then, I want to get sum = 45

Answer 1

You can put the UNION in a subquery and then SUM() the counts. But there's no need to put them in separate columns in each unioned query.

SELECT SUM(count) AS total
FROM (
    SELECT COUNT(*) AS count FROM table1
    UNION ALL
    SELECT COUNT(*) AS count FROM table2
    UNION ALL
    SELECT COUNT(*) AS count FROM table3
) AS subquery

Answer 2

to get sum of all counts from you tables you can write a query as

select 
(select COUNT(*) from table1) +
(select COUNT(*) from table2) +
(select COUNT(*) from table3) as total

Following your approach, you can wrap unioined query in as sub select and then apply sum on the results

SELECT SUM(count1) + SUM(count2) + SUM(count3) total
FROM (
    SELECT COUNT(*) AS count1, 0 AS count2, 0 AS count3 FROM table1 
    UNION ALL 
    SELECT 0 AS count1, COUNT(*) AS count2, 0 AS count3 FROM table2
    UNION ALL 
    SELECT 0 AS count1, 0 AS count2, COUNT(*) AS count3 FROM table3
) t