SELECT t1.id, t1.data, t2.c
FROM `test` AS t1
LEFT JOIN (SELECT id_test, COUNT(id_test) AS c FROM test2 GROUP BY id_test) AS t2 ON t2.id_test = t1.id
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Стимулятор ~yoomoney - 41001303250491
SELECT t1.id, t1.data, t2.c
FROM `test` AS t1
LEFT JOIN (SELECT id_test, COUNT(id_test) AS c FROM test2 GROUP BY id_test) AS t2 ON t2.id_test = t1.id