正如您所发现的，您不能从定义行级触发器的同一个表中进行选择；它会导致表变异异常。
为了使用触发器正确创建此验证，应创建一个过程以获取用户指定的锁，以便验证可以在多用户环境中正确序列化。
PROCEDURE request_lock
(p_lockname IN VARCHAR2
,p_lockmode IN INTEGER DEFAULT dbms_lock.x_mode
,p_timeout IN INTEGER DEFAULT 60
,p_release_on_commit IN BOOLEAN DEFAULT TRUE
,p_expiration_secs IN INTEGER DEFAULT 600)
IS
-- dbms_lock.allocate_unique issues implicit commit, so place in its own
-- transaction so it does not affect the caller
PRAGMA AUTONOMOUS_TRANSACTION;
l_lockhandle VARCHAR2(128);
l_return NUMBER;
BEGIN
dbms_lock.allocate_unique
(lockname => p_lockname
,lockhandle => p_lockhandle
,expiration_secs => p_expiration_secs);
l_return := dbms_lock.request
(lockhandle => l_lockhandle
,lockmode => p_lockmode
,timeout => p_timeout
,release_on_commit => p_release_on_commit);
IF (l_return not in (0,4)) THEN
raise_application_error(-20001, 'dbms_lock.request Return Value ' || l_return);
END IF;
-- Must COMMIT an autonomous transaction
COMMIT;
END request_lock;
然后可以在复合触发器中使用此过程（假设至少是 Oracle 11，这将需要在早期版本中拆分为单独的触发器）
CREATE OR REPLACE TRIGGER too_many_employees
FOR INSERT OR UPDATE ON employee
COMPOUND TRIGGER
-- Table to hold identifiers of inserted/updated employee supervisors
g_superssns sys.odcivarchar2list;
BEFORE STATEMENT
IS
BEGIN
-- Reset the internal employee supervisor table
g_superssns := sys.odcivarchar2list();
END BEFORE STATEMENT;
AFTER EACH ROW
IS
BEGIN
-- Store the inserted/updated supervisors of employees
IF ( ( INSERTING
AND :new.superssn IS NOT NULL)
OR ( UPDATING
AND ( :new.superssn <> :old.superssn
OR :new.superssn IS NOT NULL AND :old.superssn IS NULL) ) )
THEN
g_superssns.EXTEND;
g_superssns(g_superssns.LAST) := :new.superssn;
END IF;
END AFTER EACH ROW;
AFTER STATEMENT
IS
CURSOR csr_supervisors
IS
SELECT DISTINCT
sup.column_value superssn
FROM TABLE(g_superssns) sup
ORDER BY sup.column_value;
CURSOR csr_constraint_violations
(p_superssn employee.superssn%TYPE)
IS
SELECT count(*) employees
FROM employees
WHERE pch.superssn = p_superssn
HAVING count(*) > 5;
r_constraint_violation csr_constraint_violations%ROWTYPE;
BEGIN
-- Check if for any inserted/updated employee there exists more than
-- 5 employees for the same supervisor. Serialise the constraint for each
-- superssn so concurrent transactions do not affect each other
FOR r_supervisor IN csr_supervisors LOOP
request_lock('TOO_MANY_EMPLOYEES_' || r_supervisor.superssn);
OPEN csr_constraint_violations(r_supervisor.superssn);
FETCH csr_constraint_violations INTO r_constraint_violation;
IF csr_constraint_violations%FOUND THEN
CLOSE csr_constraint_violations;
raise_application_error(-20001, 'Supervisor ' || r_supervisor.superssn || ' now has ' || r_constraint_violation.employees || ' employees');
ELSE
CLOSE csr_constraint_violations;
END IF;
END LOOP;
END AFTER STATEMENT;
END;
您不需要该blocked_manager表来管理此约束。该信息可以从employee表中得出。
或者在早于 Oracle 11i 的版本中：
CREATE OR REPLACE PACKAGE employees_trg
AS
-- Table to hold identifiers of inserted/updated employee supervisors
g_superssns sys.odcivarchar2list;
END employees_trg;
CREATE OR REPLACE TRIGGER employee_biu
BEFORE INSERT OR UPDATE ON employee
IS
BEGIN
-- Reset the internal employee supervisor table
employees_trg.g_superssns := sys.odcivarchar2list();
END;
CREATE OR REPLACE TRIGGER employee_aiur
AFTER INSERT OR UPDATE ON employee
FOR EACH ROW
IS
BEGIN
-- Store the inserted/updated supervisors of employees
IF ( ( INSERTING
AND :new.superssn IS NOT NULL)
OR ( UPDATING
AND ( :new.superssn <> :old.superssn
OR :new.superssn IS NOT NULL AND :old.superssn IS NULL) ) )
THEN
employees_trg.g_superssns.EXTEND;
employees_trg.g_superssns(employees_trg.g_superssns.LAST) := :new.superssn;
END IF;
END;
CREATE OR REPLACE TRIGGER employee_aiu
AFTER INSERT OR UPDATE ON employee
IS
DECLARE
CURSOR csr_supervisors
IS
SELECT DISTINCT
sup.column_value superssn
FROM TABLE(employees_trg.g_superssns) sup
ORDER BY sup.column_value;
CURSOR csr_constraint_violations
(p_superssn employee.superssn%TYPE)
IS
SELECT count(*) employees
FROM employees
WHERE pch.superssn = p_superssn
HAVING count(*) > 5;
r_constraint_violation csr_constraint_violations%ROWTYPE;
BEGIN
-- Check if for any inserted/updated employee there exists more than
-- 5 employees for the same supervisor. Serialise the constraint for each
-- superssn so concurrent transactions do not affect each other
FOR r_supervisor IN csr_supervisors LOOP
request_lock('TOO_MANY_EMPLOYEES_' || r_supervisor.superssn);
OPEN csr_constraint_violations(r_supervisor.superssn);
FETCH csr_constraint_violations INTO r_constraint_violation;
IF csr_constraint_violations%FOUND THEN
CLOSE csr_constraint_violations;
raise_application_error(-20001, 'Supervisor ' || r_supervisor.superssn || ' now has ' || r_constraint_violation.employees || ' employees');
ELSE
CLOSE csr_constraint_violations;
END IF;
END LOOP;
END;