我想以class_id這樣的方式計算表格，它應該回傳如下
student_id | class_id | count_class_id
----------- ---------- ---------------
2222 | 22 | 2
4222 | 12 | 6
我正在寫的查詢是（下面肯定是錯誤的，需要改進）
SELECT student_id,
class_id, // I want to declare it as variable
(SELECT Count(student_id)
FROM teacher
WHERE teacher.class_id = teacher.class_id // Current Row class_id from outer of subquery ) as count_class_id
FROM teacher
uj5u.com熱心網友回復：
您需要使用兩個不同的表別名：
SELECT t1.student_id,
t1.class_id,
(SELECT Count(t2.student_id)
FROM teacher t2
WHERE t1.class_id = t2.class_id) as count_class_id
FROM teacher t1
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標籤：sql PostgreSQL
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