MySQL distinct count of data across multiple tables

You want to do a unique count of data across multiple tables.

Below is an example of getting a count of unique email addresses across two tables. Actually, I am looking for Females, aged 25 to 45 in the East Midlands, UK. I don't want duplicates because, I would not want to email the same person twice!

Code:   

SELECT COUNT(DISTINCT email) As East_Midlands
FROM (
SELECT email
FROM table_a
WHERE (gender = 'F' OR LOWER(title IN ('ms', 'miss', 'mrs')))
AND str_to_date(date, '%d/%m/%Y') BETWEEN '1967-01-01' AND '1987-08-13'
AND (SUBSTRING(postcode,1,2) IN ('CB','CO','DE','DN','IP','LE','LN','NG','NR','PE') OR postcode LIKE 'S[1-9]%')
UNION
SELECT email
FROM table_b
WHERE (gender = 'F' OR LOWER(title IN ('ms', 'miss', 'mrs')))
AND str_to_date(DOB, '%Y%m%d') BETWEEN '1967-01-01' AND '1987-08-13'
AND (SUBSTRING(postcode,1,2) IN ('CB','CO','DE','DN','IP','LE','LN','NG','NR','PE') OR postcode LIKE 'S[1-9]%')
) As SubQueryAlias;

   
I hope that makes sense. So, you have multiple selects, joined by the 'UNION', wrapped by the outer query which counts the distinct email addresses of the tables it embodies (as many as you want).

The 'As SubQueryAlias' at the end is needed to pull it together or, SQL will throw an error.