Database advanced learning notes cursor

--1. What is a cursor
The cursor ： Used to temporarily store multiple rows of data returned by a query （ Result set , Be similar to Java Of Jdbc Connection returned ResultSet aggregate ）,
By traversing the cursor , The data processing the result set can be accessed row by row .

--2. What does a cursor do
  ① Specifies the location of a specific row in the result set .
  
 ② Retrieve a row or consecutive rows based on the current result set location .

 ③ Modify the data in the row at the current position of the result set .

 ④ Define different levels of sensitivity for data changes made by other users .

 ⑤ You can access the database programmatically .

--3. Avoid using cursors
Because the efficiency of cursors is poor , If the data of cursor operation exceeds 1 Line ten thousand , Then we should rewrite ;
If a cursor is used , Try to avoid the operation of table connection in the cursor loop .

--4. Cursor classification
The cursor ： Static cursors   and   Dynamic cursors

--4.1 Static cursors
Static cursors It is divided into Show cursor   and Implicit cursors  

4.1.1 Show cursor
The user displays the declared cursor , That is, specify the result set . When the query returns more than one row , You need an explicit cursor .
It's written by users themselves sql sentence , When compiling, you can clearly know sql sentence  

4.1.2 Implicit cursors
all DML sentence （ increase 、 Delete 、 Change 、 Query single record ） Is an implicit cursor .
The variable name does not need to be declared by the user , It is defined by the system , It's called sql. There is no need to declare an implicit cursor when using , It is defined by the system  .

--4.2 Dynamic cursors
Don't know before implementation sql Statement cursor , I don't know until I execute sql The cursor of the statement .  Temporary objects for dynamically associating result sets .

--5. The syntax of cursors

The cursor ： Used to handle the use of select Statement to retrieve multiple rows of records from a database .
--（ I understand it ：select A few rows of data found will be directly returned to the user , But when the user needs to perform different operations on each piece of data returned ,
-- Receive every piece of data through the result set , Cursors, like temporary result sets, can act like cursors in result sets , Different positions that can be moved , To change ）

grammar ：cursor Cursor name ( Parameters Parameter type ) [return Return the type of cursor ] is select sentence          --( Be careful ： Need to be dealt with select sentence , Do not include INTO Clause )

Open cursor ： open Cursor name

Extract cursor ： fetch Cursor name into v_list          -- Be careful ：v_list Must be of the same type as the result set extracted by the cursor

Close cursor ： close Cursor name ;                     -- Be careful ： After closing the cursor , All resources will be released , And cannot be opened again           

How to use cursors ： Statement ---> open ---> Read ---> close

-- Be careful ：
Read cursor ：
Loop
  Fetch   Cursor name into Variable
  Exit when Cursor name %notfound
End loop;

declare
cursor cursor_fx1 is 
select ename,sal from emp;
v_ename emp.ename%type;
v_sal emp.sal%type;
begin
open cursor_fx1;
loop
fetch cursor_fx1 into v_ename,v_sal;  -- Use fetch Statement can only read one line of record
dbms_output.put_line(v_ename||v_sal);
exit when cursor_fx1%notfound;
end loop;
close cursor_fx1;
end;

-- loop   Use fetch Statement can only read one line of record , For multi row data, you need to cycle
1.while
while  t_dept%found  loop
        fetch t_dept into temp;
        dbms_output.put_line(temp.dname);
     end loop;   
2.loop
Loop
  Fetch   Cursor name into Variable
  Exit when Cursor name %notfound
End loop;
3.for       -- Automatically open the cursor , Without having to use OPEN sentence , When the cycle is over , The cursor will close automatically
for Temporary variable name in Cursor name loop
 ename:=temp_emp.ename;
          job:=temp_emp.job;
          dbms_output.put_line(ename||'  '||job);
     end loop;
     
-- Cursors with parameters
     
example ：-- Use the cursor to query and print the name and salary of employees in a department , The department number is entered manually during operation .
--loop
declare
v_ename emp.ename%type;
v_sal emp.sal%type;
cursor cursor_fx2(v_deptno emp.deptno%type) is 
select ename,sal from emp where deptno=v_deptno;
begin
open cursor_fx2(10);   ----10 It is the department number
loop
fetch cursor_fx2 into v_ename,v_sal;
dbms_output.put_line(v_ename||v_sal);
exit when cursor_fx2%notfound ;
end loop;
close cursor_fx2;
end;

-for
declare
v_ename varchar2(20);  --- Be careful
v_sal varchar2(20);
cursor cursor_fx3(v_deptno emp.deptno%type) is 
select ename,sal from emp where deptno=v_deptno;
begin
for xh1 in cursor_fx3(10) loop
v_ename:=xh1.ename;
v_sal:=xh1.sal;
dbms_output.put_line(v_ename||v_sal); 
end loop;
end;