declare：用於plsql中的宣告變數，和begin連用，和儲存過程結構相似

declare
   v_sal  number(7,2):=60000;--宣告變數v_sal 型別為number(7,2)前面的7表示總共允許7個字元長，稱為寬度。後面2是保留小數點後面兩位，稱為精度。
   v_comm2 number(7,2):=v_sal*0.20;
   v_aa number(7,2):=100;
   v_message varchar2(255):='eligible for commission';
begin
   declare
         v_sal   number(7,2):=10;
         v_comm  number(7,2):=0;
         v_total_comp number(7,2):=v_sal+v_comm;
    begin
         v_message:='clerk not'||v_message;   
         v_comm:=v_sal*0.30;
         v_aa:=v_aa*0.30;
         DBMS_OUTPUT.PUT_LINE ('2:'||TO_CHAR(v_aa));--dbms_output.put_line 輸出資料括號裡的||代表字串的拼接
         
         DBMS_OUTPUT.PUT_LINE ('1:'||TO_CHAR(v_comm2));
     end;
         v_message:='salesman'||v_message;
         DBMS_OUTPUT.PUT_LINE (TO_CHAR(v_comm2));
end;

select employee_ID as employees,first_name||last_name as "employee Name",Department_ID as "departement",salary as "Salary",email as "Email",phone_number as "phone",hire_date as "ehire date",job_id as "job Id" from employees;

select employee_ID  employees,first_name||last_name  "employee Name",Department_ID  "departement",salary  "Salary",email  "Email",phone_number  "phone",hire_date  "ehire date",job_id  "job Id" from employees;

以上兩句都能實現以下效果，plsql中取別名最好不要用as，||符號是拼接字串