Oracle SQL - 我可以在 DDL 的 CHECK 约束中使用子查询吗?
Oracle SQL - Can I use sub-query in a CHECK constraint in DDL?
我正在使用 Oracle SQL 我想知道当我创建一个新的 table.请考虑以下示例:
假设我想创建一个名为 EnrollsIn
的 table,它有两个属性 - student
和 course
。 student
是一个包含唯一学生 ID 的字符串,course
属性包含一串课程代码。
CREATE TABLE EnrollsIn (
student char(8) PRIMARY KEY,
course char(7),
-- Insert constraint here (Constraint is written below)
);
INSERT INTO EnrollsIn VALUE ('12345678', 'COMP200');
INSERT INTO EnrollsIn VALUE ('12345678', 'COMP300');
INSERT INTO EnrollsIn VALUE ('12345678', 'COMP400');
INSERT INTO EnrollsIn VALUE ('12345678', 'MATH100'); -- This violates the constraint written below!
现在,我希望这个 table 有一个限制,即一个学生最多只能注册 3 门课程。换句话说,对于每个 student
:
,以下查询应始终具有 enrollCount
<= 3 的值
-- I want the value "enrollCount" always <= 3 for all students
SELECT student, count(*) as enrollCount FROM EnrollsIn GROUP BY student;
在 CERATE TABLE EnrollsIn
语句中定义 CHECK 约束时,我可以使用这个子查询吗?如果是这样,我该如何定义这个约束?
最好向documentation询问一下:
Restrictions on Check Constraints
Check constraints are subject to the following restrictions:
- Conditions of check constraints cannot contain the following constructs:
-
- Subqueries and scalar subquery expressions
-
- Calls to user-defined functions
我正在使用 Oracle SQL 我想知道当我创建一个新的 table.请考虑以下示例:
假设我想创建一个名为 EnrollsIn
的 table,它有两个属性 - student
和 course
。 student
是一个包含唯一学生 ID 的字符串,course
属性包含一串课程代码。
CREATE TABLE EnrollsIn (
student char(8) PRIMARY KEY,
course char(7),
-- Insert constraint here (Constraint is written below)
);
INSERT INTO EnrollsIn VALUE ('12345678', 'COMP200');
INSERT INTO EnrollsIn VALUE ('12345678', 'COMP300');
INSERT INTO EnrollsIn VALUE ('12345678', 'COMP400');
INSERT INTO EnrollsIn VALUE ('12345678', 'MATH100'); -- This violates the constraint written below!
现在,我希望这个 table 有一个限制,即一个学生最多只能注册 3 门课程。换句话说,对于每个 student
:
enrollCount
<= 3 的值
-- I want the value "enrollCount" always <= 3 for all students
SELECT student, count(*) as enrollCount FROM EnrollsIn GROUP BY student;
在 CERATE TABLE EnrollsIn
语句中定义 CHECK 约束时,我可以使用这个子查询吗?如果是这样,我该如何定义这个约束?
最好向documentation询问一下:
Restrictions on Check Constraints
Check constraints are subject to the following restrictions:
- Conditions of check constraints cannot contain the following constructs:
- Subqueries and scalar subquery expressions
- Calls to user-defined functions