What Does "<>" Mean in Oracle

What does mean in Oracle

It means 'not equal to'. So you're filtering out records where ordid is 605. Overall you're looking for any records which have the same prodid and qty values as those assigned to ordid 605, but which are for a different order.

What does || ' ' || mean in oracle SQL

The SQL || operator allows you to concatenate 2 or more strings together.

Example:
varchar 1: Harry -
Varchar 2: Mueller

You can concatenate this two with || operator:

Select 'Harry' || 'Mueller' from dual:

output:

HarryMueller

But with a space you can seprate these two. You can add a space between two varchar with a ' ' :

Select 'Harry' || ' ' || 'Mueller' from dual:

output:

Harry Mueller

What is the meaning of '@' symbol in Oracle SQL?

It refers to a non-local table, the bit behind the @ is the db descriptor.

select * from question_answer@abcd where id = '45'

Means select not from the local question_answer table, but from the table on the db designated as abcd. The keyword to google for is dblink

What does := mean in oracle when we use it

:= is the assignment operator in PL/SQL (Oracle's procedural extension to SQL). You use this to assign values to variables. If you just use = then this is checking for equality rather than assigning a value.

Here is a very simple example using the assignment operator to assign values to variables:

Declare
   v1 number;
   v2 number;
   res number;
Begin
   --initialise values
   v1 := 2;
   v2 := 2;
   res := v1 + v2;
   dbms_output.put_line(res);
end;

I think you will need to be a bit more specific about what you want to know about dynamic SQL. As the comment above suggests, it would also be best to raise one thread per question as these are unrelated.

What does a (+) sign mean in an Oracle SQL WHERE clause?

This is an Oracle-specific notation for an outer join. It means that it will include all rows from t1, and use NULLS in the t0 columns if there is no corresponding row in t0.

In standard SQL one would write:

SELECT t0.foo, t1.bar
  FROM FIRST_TABLE t0
 RIGHT OUTER JOIN SECOND_TABLE t1;

Oracle recommends not to use those joins anymore if your version supports ANSI joins (LEFT/RIGHT JOIN) :

Oracle recommends that you use the FROM clause OUTER JOIN syntax rather than the Oracle join operator. Outer join queries that use the Oracle join operator (+) are subject to the following rules and restrictions […]

Oracle: What does this expression mean in a SQL query?

It means not equal to . It's the same as != in C-like languages.

PL/SQL What's the meaning of word

As mentioned by the question commenters, this is used to label compound statements and also as a target for GOTO. You can use to use the label in the END and END LOOP which can be great for readability, like <<countdown>> for i in 1.9 loop blah; end loop countdown;

Here is an example:

set echo on
set serveroutput on

<<begin_end_block>> 
declare 
  msg varchar2(1000);
begin
  dbms_output.enable;
  msg := chr(9) || 'start';
  <<loopblk>> 
  for itr8 in 1 .. 5
  loop
    msg := msg || chr(10) || chr (9) ||  'loop';
    dbms_output.put_line ('Iterator is ' || itr8);
    <<ifblck>> if itr8 > 2
    then
      msg := msg || chr(10) || chr(9) || 'greater than 2';
      goto gototarg;
    end if;
    exit loopblk when mod (itr8, 4) = 0;
    continue loopblk;
    <<gototarg>>
    dbms_output.put_line ('after goto target');
  end loop loopblk;
  dbms_output.put_line ('Ending, here are the messages' || chr(10) || msg);
end begin_end_block;
/

output is:

anonymous block completed
Iterator is 1
Iterator is 2
Iterator is 3
after goto target
Iterator is 4
after goto target
Iterator is 5
after goto target
Ending, here are the messages
    start
    loop
    loop
    loop
    greater than 2
    loop
    greater than 2
    loop
    greater than 2

Oracle (+) Operator

That's Oracle specific notation for an OUTER JOIN, because the ANSI-89 format (using a comma in the FROM clause to separate table references) didn't standardize OUTER joins.

The query would be re-written in ANSI-92 syntax as:

   SELECT ...
     FROM a
LEFT JOIN b ON b.id = a.id

This link is pretty good at explaining the difference between JOINs.

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It should also be noted that even though the (+) works, Oracle recommends not using it:

Oracle recommends that you use the FROM clause OUTER JOIN syntax rather than the Oracle join operator. Outer join queries that use the Oracle join operator (+) are subject to the following rules and restrictions, which do not apply to the FROM clause OUTER JOIN syntax:

What does @+ and @* mean in (oracle) SQL?

As mentioned by a_horse_with_no_name:
"That's invalid in Oracle. Looks like a placeholder that gets replaced by some tool"
It is in fact replaced by some internal proprietary tool.

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