PLSQL_（3）程序控制结构

pl/sql语言是oracle对关系数据库语言sql的过程化扩充。
一、顺序结构

declare
v_student students%rowType;
begin
select * into v_student
from students
where student_id = 10213;
dbms_output.put_line('姓名：'|| v_student.name);
dbms_output.put_line('性别：'|| v_student.sex);
dbms_output.put_line('专业：'|| v_student.specialty);
end;

二、分支结构
1.if语句
语法格式：

if condition_1 then
statements_1;
...
[elseif condition_n then
statements_n;]
[else
else statements;]
end if;
declare
v_id teachers.teacher_id%type;
v_title teachers.title%type;
begin
v_id := &teacher_id;
select title into v_title
from teachers
where teacher_id = v_id;
if v_title = '讲师' then
update teachers
set wage = 1.1 * wage
where teacher_id = v_id;
elsif v_title = '高工' or v_title = '副教授' then
update teachers
set wage = 1.05 * wage
where teacher_id = v_id;
else
update teachers
set wage = wage + 100
where teacher_id = v_id;
end if;
end;

2.case语句
使用if语句处理复杂的分支（多分支）操作，程序结构不够清晰，往往会给编制程序、调试程序
、阅读程序带来一定的困难。使用case语句可以使程序结构比较清晰。
（1）等值比较的case语句
语法格式：

case expression
when result_1 then
statements_1;
when result_2 then
statements_2;
...
[else
else_statements;]
end case;

当使用case语句执行多重分支时，如果条件表达式完全相同，并且条件表达式为相等条件选择，
那么可以选择使用等值比较的case语句。

declare
v_id teachers.teacher_id%type;
v_title teachers.title%type;
begin
v_id := &teacher_id;
select title into v_title
from teachers
where teacher_id = v_id;
case v_title
when '教授' then
update teachers
set wage = 1.5 * wage
where teacher_id = v_id;
when '高工' then
update teachers
set wage = 1.1 * wage
where teacher_id = v_id;
else
update teachers
set wage = wage + 100
where teacher_id = v_id;
end case;
end;

（2）多种条件比较的case语句
语法格式：

case
when expression1 then
statements_1;
when expression2 then
statements_2;
...
[else
else_statements;]
end case;

如果条件表达式不相同，或条件表达式不完全为相等条件选择，那么需要选择使用多种条件比较的case语句，相当于多个IF的语句。

declare
v_id teachers.teacher_id%type;
v_title teachers.title%type;
begin
v_id := &teacher_id;
select title into v_title
from teachers
where teacher_id = v_id;
case
when v_title = '教授' then
update teachers
set wage = 1.5 * wage
where teacher_id = v_id;
when v_title =  '高工' or v_title =  '副教授' then
update teachers
set wage = 1.1 * wage
where teacher_id = v_id;
else
update teachers
set wage = wage + 100
where teacher_id = v_id;
end case;
end;

declare
v_id teachers.teacher_id%type;
v_bonus teachers.bonus%type;
v_eage teachers.wage%type;
v_income number(7,2);
begin
v_id := &teacher_id;
select bonus, wage into v_bonus, v_wage
from teachers
where teacher_id = v_id;
v_income := v_bonus + v_wage;
case
when v_income <= 1000 then
dbms_output.out_line('个人所得税：0');
when v_income > 1000 and v_income < 3000 then
dbms_output.out_line('个人所得税：'||v_income*0.03);
when v_income >= 3000 then
dbms_output.out_line('个人所得税：'||v_income*0.05);
end case;
end;

三、循环结构
1.loop循环
语法格式：

loop
statements(s);
exit [when condition];
end loop;
create table total(n int, result int);
declare
v_i int := 1;
v_sum int :=0;
begin
loop
v_sum := v_sum + v_i;
insert into total values(v_i, v_sum);
exit when v_i = 10;
end loop;
end;

2.while循环
语法格式：

while condition loop
statement(s);
end loop;
declare
v_i int := 1;
v_sum int := 0;
begin
while v_i <= 10 loop
v_sum := v_sum + v_i * v_i;
insert into total values(v_i, v_sum);
v_i := v_i + 1;
end loop;

3.for循环
loop循环和while循环需要定义循环控制变量来控制循环次数；然而for循环不需要定义循环控制变量，
系统默认定义一个循环控制变量，以控制循环的次数。
语法格式：

for loop_index in [reverse] lowest_number ..highest_number loop
statement(s);   --[reverse]表示循环控制变量从上限向下限递减1.
end loop;
declare
v_i int := 1;
v_factorial int := 1;
begin
for v_i in 1..10 loop
v_factorial := v_factorial * v_i;
insert into total values(v_i, v_factorial);
end loop
end;

4.goto语句与Null语句
（1）goto语句
--goto语句用于改变Pl/sql程序的执行顺序。
语法格式：
goto label_name; --pl/sql程序无条件地跳转到标号label_name处执行。
注意：标号后至少要有一条可执行语句；
pl/sql块内部可以跳转，内层块可以跳到外层块，但外层块不能跳到内层块；
不能从某一if语句外部跳到其内部；
不能从某一循环体外跳到体内；

set serveroutput on
declare
v_i int := 1;
v_sum int := 0;
begin
loop
v_sum := v_sum + v_i;
insert into total values(v_i, v_sum);
if v_i = 10 then
goto output;
end if;
v_i := v_i + 1;
end loop;
<<output>>
dbms_output.put_line('v_sum = ' || v_sum);
end;

不能从某一子程序外部跳到其内部。

（2）null语句
--null语句被称为空语句，它不执行任何操作便将程序控制交给下一条语句。
语法格式：
null:
--使用null语句可以提高程序的可读性，一般在标号之后或在异常处理程序段中使用。

declare
v_i int := 1;
v_sum int := 0;
begin
loop
v_sum := v_sum + v_i;
insert into total values(v_i, v_sum);
if v_i = 10 then
goto output;
end if;
v_i := v_i + 1;
end loop;
<<output>>
null;
end;