oracle触发器在插入更新后计算一个表

Question

我在mysql中有这个触发器工作正常：

CREATE TRIGGER ins_trainee AFTER INSERT ON trainee
FOR EACH ROW UPDATE training_activities SET NO_of_Trainees = 
( SELECT COUNT(SN)
FROM trainee
WHERE trainee.training_type = training_activities.SN
)

我试着像这样转换到oracle：

CREATE OR REPLACE TRIGGER ins_trainee
AFTER INSERT ON trainee
FOR EACH ROW 

BEGIN
UPDATE training_activities SET NO_of_Trainees =
SELECT COUNT(SN)
FROM trainee
WHERE trainee.training_type = training_activities.SN;

END;
/

但我得到warning: trigger created with compilation errors

show errors给出了：

2/1     PL/SQL: SQL Statemnet Ignored
2/49    PL/SQL: ORA-00936: missing expression