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【ORACLE】21c版本新特性之for...loop循环的增强

DarkAthena
2021-11-24 / 0 评论 / 0 点赞 / 679 阅读 / 10104 字

前言

接上篇
【ORACLE】18c版本的限定表达式及其在21c版本的增强

上篇最后说到了迭代器,由于内容太多,所以和本篇放一起了。

在迭代器中,除了上篇说的三种迭代器(基本、索引、序列)外,还支持多种关键词组合使用。

类似"for i in 1..10" 这种,被称之为迭代器(iterator),既可以用于限定表达式,也可以用于for loop循环,是共用的语法,所以本篇不会对每种用法都分别写两种例子,读者可自行尝试修改成限定表达式的写法或者for loop的写法。

老规矩,先上官方文档
https://docs.oracle.com/en/database/oracle/oracle-database/21/lnpls/plsql-control-statements.html#GUID-C4BC9960-5945-4646-BBDE-DC00346F8702

for loop语法

[ label ] for_loop_header 
  statements
END LOOP [ label ];

for_loop_header ::= FOR iterator LOOP

iterator ::= iterand_decl [, iterand_decl] IN iteration_ctl_seq 

iterand_decl  ::= pls_identifier [ MUTABLE | IMMUTABLE ] [ constrained_type ]          

iteration_ctl_seq ::= qual_iteration_ctl [,]...

qual_iteration_ctl ::= [ REVERSE ] iteration_control  pred_clause_seq

iteration_control ::= stepped_control 
                      | single_expression_control 
                      | values_of_control 
                      | indices_of_control 
                      | pairs_of_control  
                      | cursor_control 

pred_clause_seq ::= [ stopping_pred ] [ skipping_pred ]

stopping_pred ::= WHILE boolean_expression  

skipping_pred ::= WHEN boolean_expression 

stepped_control ::= lower_bound .. upper_bound [ BY step ]

single_expression_control ::= [ REPEAT ] expr

下面这段官方可能是写错了,“cursor_iteration__control”其实对应上面的“cursor_control”,而且还存在两个明显的输入错误 "_"重复了,"cursor _object"多了一个空格

cursor_iteration__control ::=  { cursor _object
                    | sql_statement
                    | cursor_variable
                    | dynamic_sql }

qual_iteration_ctl

REVERSE

"REVERSE"(不是21c添加的),为对后面的元素进行倒序排列

begin
  for i in REVERSE 1 .. 5  loop
    dbms_output.put_line ( i );
  end loop;
end;
/
---
5
4
3
2
1

iteration_control

BY (stepped_control)

先看例子

SET SERVEROUTPUT ON;

DECLARE
  TYPE t_tab IS TABLE OF VARCHAR2(10) INDEX BY pls_integer;
  l_tab t_tab := t_tab(for i in 1..10 by 2  sequence => i);
BEGIN
  FOR I IN 1 .. l_tab.COUNT LOOP
    DBMS_OUTPUT.put_line(i||':'|| l_tab(I));
  END LOOP;
END;
/

---
1:1
2:3
3:5
4:7
5:9

"by" 在 for loop 中的使用

begin
  for i in 1 .. 10 by 2 loop
    dbms_output.put_line ( i );
  end loop;
end;
/

---
1
3
5
7
9

"by"表示对 in 的元素进行间隔选择(Step(步长)),比如上例中的2,就是每次选择向后第2个元素,所以依次是第1、3、5、7、9个元素。

single_expression_control

REPEAT

"REPEAT",按照后面的表达式循环产生元素

BEGIN
   FOR i IN 1, REPEAT i*2 WHILE i < 100 LOOP
      DBMS_OUTPUT.PUT_LINE(i);
   END LOOP;
END;
/
---
1
2
4
8
16
32
64

VALUES OF (values_of_control)

取出数组中的值

DECLARE
   TYPE intvec_t IS TABLE OF PLS_INTEGER INDEX BY PLS_INTEGER;
   vec intvec_t := intvec_t(3 => 10, 1 => 11, 100 => 34);
BEGIN
   FOR i IN VALUES OF vec LOOP
      DBMS_OUTPUT.PUT_LINE(i);
   END LOOP;
END;
/
---
11
10
34

INDICES OF (indices_of_control )

取出数组中的索引(键)

DECLARE
   TYPE intvec_t IS TABLE OF PLS_INTEGER INDEX BY PLS_INTEGER;
   vec intvec_t := intvec_t(3 => 10, 1 => 11, 100 => 34);
BEGIN
   FOR i IN INDICES OF vec LOOP 
       DBMS_OUTPUT.PUT_LINE(i);  
   END LOOP;
END;
/
---
1
3
100

PAIRS OF(pairs_of_control )

取出 索引和值 的对(键值对)

DECLARE
   TYPE intvec_t IS TABLE OF PLS_INTEGER INDEX BY PLS_INTEGER;
   vec  intvec_t := intvec_t(3 => 10, 1 => 11, 100 => 34);
BEGIN
   FOR i,j IN PAIRS OF vec LOOP
      DBMS_OUTPUT.PUT_LINE(i || '=>'|| j);
   END LOOP;
END;
/
---
1=>11
3=>10
100=>34

cursor_control

sql_statement

直接用一个查询sql(不是21c添加的)

begin
  for rec in (select a.region_id, a.region_name from hr.regions a) loop
    dbms_output.put_line(rec.region_id || '-' || rec.region_name);
  end loop;
end;
/

cursor _object

先用一个查询sql定义一个游标对象,然后直接使用这个对象(不是21c添加的)

declare
  cursor c is
    select a.region_id, a.region_name from hr.regions a;
begin
  for rec in c loop
    dbms_output.put_line(rec.region_id || '-' || rec.region_name);
  end loop;
end;
/

cursor_variable

即 ref cursor动态游标(不是21c添加的)

declare
  l_sql varchar2(4000):='select * from hr.regions';
  l_row hr.regions%rowtype;
  type cur is ref cursor;
  curs cur;
begin
  open curs for l_sql;
  loop
    fetch curs into l_row;
    exit when curs%notfound;
    dbms_output.put_line(l_row.region_name);
    end loop;
    close curs;
end;
/

dynamic_sql

即动态sql,且支持绑定变量

declare
  l_sql varchar2(4000) := 'select * from hr.regions where REGION_ID<=:1';
begin
  for r hr.regions%rowtype in (execute immediate l_sql using 3) loop
    dbms_output.put_line(r.region_name);
  end loop;
end;
/

动态sql是最能让人发挥想象力之一的了,可惜的是被这个限定type(constrained_type)限制住了。上例中,如果不定义r的type,或者用非限定type,像下面这样,会报错

declare
  l_sql varchar2(4000) := 'select * from hr.regions where REGION_ID<=:1';
begin
  for r in (execute immediate l_sql using 3) loop
    null;
  end loop;
end;
/

ORA-06550: 第 4 行, 第 13 列:
PLS-00858: 没有为 EXECUTE IMMEDIATE 迭代控制指定 iterand 类型。
ORA-06550: 第 4 行, 第 3 列:
PL/SQL: Statement ignored

另外要注意的是,动态sql的这种方式不能用于限定表达式定义的默认值,只能在begin后再进行赋值。

pred_clause_seq

WHEN (skipping_pred )

"WHEN" 为只选择出满足后面表达式的元素

BEGIN
   FOR i IN 1..10 when mod(i,2) =0 LOOP
      DBMS_OUTPUT.PUT_LINE(i);
   END LOOP;
END;
/

---
2
4
6
8
10

WHILE (stopping_pred )

"WHILE"为直到不满足后面条件则停止选择元素

BEGIN
   FOR i IN 1..10  WHILE i<=5 LOOP
      DBMS_OUTPUT.PUT_LINE(i);
   END LOOP;
END;
/

---
1
2
3
4
5

iteration_ctl_seq ::= qual_iteration_ctl [,]...

这语法其实表示,我们可以把多个qual_iteration_ctl用逗号隔开组合成一个iteration_ctl_seq,比如

DECLARE
   TYPE intvec_t IS TABLE OF PLS_INTEGER INDEX BY PLS_INTEGER;
   vec intvec_t := intvec_t(3 => 10, 1 => 11, 100 => 34);
   l_sql varchar2(4000):='select a.region_id from hr.regions a';
BEGIN
   FOR i IN 1..6 by 2, 
            REVERSE i+1..i+5,
            repeat i*2 while i<20,
            1..30 when mod(i,9)=0 , 
            values of vec,
            INDICES OF vec,
            vec.first,
            vec.last-1,
            values of (execute immediate l_sql) LOOP
      DBMS_OUTPUT.PUT_LINE(i);
   END LOOP;
END;
/
---
1
3
5
10
9
8
7
6
12
9
18
27
11
10
34
1
3
100
1
99
1
2
3
4

或者改成限定表达式的形式

DECLARE
   TYPE intvec_t IS TABLE OF PLS_INTEGER INDEX BY PLS_INTEGER;
   vec intvec_t := intvec_t(3 => 10, 1 => 11, 100 => 34);
   vec2 intvec_t;
   l_sql varchar2(4000):='select a.region_id from hr.regions a';
BEGIN
  vec2 :=intvec_t(FOR i IN  1..6 by 2, 
                            REVERSE i+1..i+5,
                            repeat i*2 while i<20,
                            1..30 when mod(i,9)=0 ,
                            values of vec,
                            INDICES OF vec,
                            vec.first,
                            vec.last-1,
                            values of (execute immediate l_sql)
                            sequence=>i);
   for i in 1..vec2.count loop
     DBMS_OUTPUT.PUT_LINE(vec2(i));
     end loop;
END;
/

这个例子的iterand_decl其实就是由下面这九个qual_iteration_ctl共同组成

  • "1..6 by 2",
  • "REVERSE i+1..i+5",
  • "repeat i*2 while i<20",
  • "1..30 when mod(i,9)=0 ",
  • "values of vec",
  • "INDICES OF vec"
  • "vec.first"
  • "vec.last-1"
  • "values of (execute immediate l_sql)"

可以看到这里用了前文没提到的first和last,其实这是collection对象的函数,与之类似的还有count、limit、prior、next等。由于不涉及本篇主要内容,就不继续扩展了,关于集合的更多函数请参考官方文档
https://docs.oracle.com/en/database/oracle/oracle-database/21/lnpls/plsql-collections-and-records.html#GUID-0452FBDC-D9C1-486E-B432-49AF84743A9F

总结

从18c到21c各种限定表达式及for...loop循环的增强,再配合集合的各种操作,使得plsql编程语法变得更宽松,可以大大提高编程效率。

更多例子可参考下面这篇文章
Better loops and qualified expressions (array constructors) in PL/SQL

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