Вы можете попробовать это, если ваша база данных MySQL или SQL-сервер. Для других баз данных вы можете использовать логику / идею.
SELECT
SUM(CASE WHEN Type = 'J' THEN C ELSE 0 END) - SUM(CASE WHEN Type = 'L' THEN C ELSE 0 END) AS [Total],
SUM(CASE WHEN Type = 'L' THEN C ELSE 0 END) AS [left],
SUM(CASE WHEN Type = 'J' THEN C ELSE 0 END) AS [join],
M Month,
Y Year
FROM
(
SELECT 'J' AS [Type],
MONTH(CONVERT(DATETIME, [Join], 103)) M,
YEAR(CONVERT(DATETIME, [Join], 103)) Y,
COUNT(ID) C
FROM customers
GROUP BY MONTH(CONVERT(DATETIME, [Join], 103)), YEAR(CONVERT(DATETIME, [Join], 103))
UNION ALL
SELECT 'L',
MONTH(CONVERT(DATETIME, [left], 103)) M,
YEAR(CONVERT(DATETIME, [left], 103)) Y,
COUNT(ID) C
FROM customers
GROUP BY MONTH(CONVERT(DATETIME, [left], 103)), YEAR(CONVERT(DATETIME, [left], 103))
)A
WHERE Y IS NOT NULL
GROUP BY M,Y