减去两个表之间的计数

Question

我正在使用MySQL

如何从另一个表中减去记录来计算。 例如，count = tab1 - tab2

表：tab1

+-------------+----------------+
| studnetId   +   batchId      +
+-------------+----------------+
+    1        +      1         +
+-------------+----------------+
+    2        +      1         +
+-------------+----------------+
+    3        +      1         +
+-------------+----------------+
+    4        +      1         +
+-------------+----------------+
+    5        +      2         +
+-------------+----------------+
+    6        +      2         +
+-------------+----------------+
+    7        +      2         +
+-------------+----------------+

表：tab2

+-------------+----------------+
| studnetId   +   batchId      +
+-------------+----------------+
+    1        +      1         +
+-------------+----------------+

预期结果

+-------------+----------------+
| count       +   batchId      +
+-------------+----------------+
+    3        +      1         +
+-------------+----------------+
+    2        +      2         +
+-------------+----------------+

Answer 1

如果我理解正确，您可以使用not exists：

select count(*), batchid
from tab1 t1
where not exists (select 1
                  from tab2 t2
                  where t2.studnetid = t1.studnetid and t2.batchid = t1.batchid
                 );