Oracle SQL 将 sql 值设置为变量

Question

I am trying to compare dates between the same table. I wanted to store each value to a variable and compare the if there is a month difference between the two dates.

Here is my table structure:

TABLE NAME: SYSTEMPARAM
SEQNO - INT primary key
SYSTEMDATE - DATE

Here is what I want to achieve:

var startdate = "SELECT SYSTEMDATE FROM SYSTEMPARAM WHERE SEQNO = '1'";
var enddate = "SELECT SYSTEMDATE FROM SYSTEMPARAM WHERE SEQNO = '2'";

if(MONTHS_BETWEEN(startdate , enddate ){

  //Conditions here
}

I want to compare if there is a month difference between two sql statement.

Answer 1

"Procedural" means PL/SQL. So, do it that way.

Just to know what dates represent:

SQL> alter session set nls_date_format = 'dd.mm.yyyy';

Session altered.

Table contents:

SQL> select * From systemparam;

     SEQNO SYSTEMDATE
---------- ----------
         1 04.04.2021
         2 05.04.2021

Anonymous PL/SQL block:

SQL> set serveroutput on
SQL> declare
  2    l_startdate systemparam.systemdate%type;
  3    l_enddate   systemparam.systemdate%type;
  4  begin
  5    select s.systemdate
  6      into l_startdate
  7      from systemparam s
  8      where s.seqno = 1;
  9
 10    select s.systemdate
 11      into l_enddate
 12      from systemparam s
 13      where s.seqno = 2;
 14
 15    if months_between(l_enddate, l_startdate) > 0 then
 16       dbms_output.put_line('Dates are not equal');
 17    end if;
 18  end;
 19  /
Dates are not equal

PL/SQL procedure successfully completed.

SQL>

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Though, if there are any "months" between these two dates ("0.01" months between is still "months between", although tiny ), then they aren't equal so it is maybe more obvious to use if l_enddate <> l_startdate then instead, isn't it? Also, check another option of the SELECT statement:

SQL> declare
  2    l_startdate systemparam.systemdate%type;
  3    l_enddate   systemparam.systemdate%type;
  4  begin
  5    select max(case when s.seqno = 1 then s.systemdate end),
  6           max(case when s.seqno = 2 then s.systemdate end)
  7      into l_startdate,
  8           l_enddate
  9    from systemparam s;
 10
 11    if l_enddate <> l_startdate then
 12       dbms_output.put_line('Dates are not equal');
 13    end if;
 14  end;
 15  /
Dates are not equal

PL/SQL procedure successfully completed.

SQL>

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