Question

I have a mysql db/server that has 3 tables that are identical in structure:
west, midwest and east.

I would like to create a national table with the sum of the columns of those regional tables, ONLY if the datetime row matches all 3 tables. That way if one hour is missing in a particular table, I don’t end up summing 2 regions and calling it national.

Here is how I am thinking to do it:

All 3 tables have a datetime column.

Merge the tables (union?) only if the datetime row exists in all 3 tables.

Aggregate (sum) the columns grouped by datetime column. I would of course be summing all columns which carry int values.

I am not sure how to run a query that would perform this task.
These tables have 11mil rows so an efficient way would be great.
I am also open to other approaches to solve this problem.

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I picked the answer from Neil because although the answer would not work if datetime col is not unique i.e. multiple rows in Table1 with the same datetime. Using any other method the performance I got was horrific, hours of query time. I decided to compromise. I created 3 new tables
westh, midwesth and southh.
These 3 new tables are a creation of aggregating the original tables by hour.
I then used Neils second version with a twist:

INNER JOIN Table2 USING (datetime)

While datetime is indexed in my tables that provides superior performance which is a firm criteria for me.

Answer 1

First version:

SELECT T123.dtcol, SUM(T123.intcol) AS intcolsum
  FROM (
    SELECT Table1.dtcol, Table1.intcol FROM Table1
    UNION
    SELECT Table2.dtcol, Table2.intcol FROM Table2
    UNION
    SELECT Table3.dtcol, Table3.intcol FROM Table3
  ) T123
  GROUP BY T123.dtcol
  HAVING COUNT(*) = 3

Second version:

SELECT Table1.dtcol, Table1.intcol + Table2.intcol + Table3.intcol AS intcolsum
  FROM Table1 T1
  INNER JOIN Table2 T2 ON T2.dtcol = T1.dtcol
  INNER JOIN Table3 T2 ON T3.dtcol = T1.dtcol