各個資料庫軟體對於sql的支援
最近正在學習資料庫這門課程，雖然sql有有統一的標準，但是各個資料庫軟體對於sql支援是不一樣的，上機遇到各種各樣的問題，就記錄在這個帖子上吧；
1，mysql是不支援對檢視建立觸發器的；
2，oracle只支援了級聯刪除on delete cascade卻不支援級聯更新on update cascade；
3，mysql中的條件控制語句比如if語句後是可以出現查詢語句selec的，但是oracle中是不可以的，只能實現begin開始前宣告變數，然後再begin語句後把查詢出來的的結果賦給變數，在oracle中條件控制語句是用when 的
4，mysql中新插入，修改後的行用new來表示，而刪除或者更新前的行用old來表示，而在oracle中則需要在nrow前面加上：號，還要在開頭加上一句referencing new as nrow；
但是在when後面的nrow是不用加上：號的；
下面是今天晚上的幾個例子（資料庫使用的是oracle）：
[sql]
create database student;
create table students
(sno varchar(15),
sname varchar(20) not null,
*** varchar(7),
bdate date,
height int,
department varchar(20),
primary key (sno));
create table courses
(cno varchar(8),
cname varchar(20) not null,
lhour int,
credit int,
semester varchar(6),
primary key (cno));
create table sc
(sno varchar(15),
cno varchar(8),
grade smallint,
primary key (sno,cno),
foreign key (sno) references students(sno)
on delete cascade,
foreign key (cno) references courses(cno)
on delete cascade);
create or replace trigger upd_creadits after insert on sc
referencing new as nrow for each row
declare credits_count int;
credits_number int;
begin
select count(*) into credits_count from credits where credits.sno=:nrow.sno;
select credit into credits_number from courses where courses.cno=:nrow.cno;
if :nrow.grade>=60 then
if credits_count<>0 then
update credits
set sumcredit=sumcredit+credits_number
where sno=:nrow.sno;
else
insert into credits values(:nrow.sno,credits_number,0);
end if;
else
if credits_count<>0 then
update credits
set nopass=1+nopass
where sno=:nrow.sno;
else
insert into credits values(:nrow.sno,0,1);
end if;
end if;
end;
/
create or replace trigger upd_stuview instead of insert on student_grade
referencing new as nrow for each row
declare student_num int;
course_num int;
xuehao varchar(15);
kechenghao varchar(8);
begin
begin
select count(*) into student_num from students where students.sname=:nrow.sname;
exception when no_data_found then
dbms_output.put_line('不存在這個學生或者這門課程!');
end;
begin
select count(*) into course_num from courses where courses.cname=:nrow.cname;
exception when no_data_found then
dbms_output.put_line('不存在這個學生或者這門課程!');
end;
select sno into xuehao from students where students.sname=:nrow.sname;
select cno into kechenghao from courses where courses.cname=:nrow.cname;
if student_num<>0 and course_num<>0 then
insert into sc values(xuehao,kechenghao,:nrow.grade);
else dbms_output.put_line('不存在這個學生或者這門課程!');
end if;
end;
下面是第乙個觸發器upa_creadits的mysql版本：
delimiter |
create trigger upd_creadits after insert on sc
for each row
begin
if new.grade>=60 then
if exists(select * from credits where credits.sno=new.sno) then
update credits
set sumcredit=sumcredit+(select credit from courses where courses.cno=new.cno)
where sno=new.sno;
else
insert into credits values(new.sno,(select credit from courses where courses.cno=new.cno),0);
end if;
else
if exists(select * from credits where credits.sno=new.sno) then
update credits
set nopass=1+nopass
where sno=new.sno;
else
insert into credits values(new.sno,0,1);
end if;
end if;
end|
mysql不支援檢視觸發器；
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