What does where 1=1 mean in sql code

Question

I am going throough someones sql code and they have used where 1=1 in alot of places what is this for... it doesnt make sense to me...

Answer 1

"create table emp1 as select * from emp where 1=1"
While your statement that this will create a copy of the emp table is true, the predicate is meaningless.
CREATE TABLE emp1 AS
SELECT * FROM empwill do the exact same thing with less typing.
Puppethead
The same argument applies to your use of the predicate in development. WHERE 1 = 1 does not change the meaning of the query, no matter how many other predicates you add or remove from the query. The result of the query will be governed by the meaningful predicates in the WHERE clause.
WHERE 1 = 1 is noise, and is ignored by the optimizer.
SQL> EXPLAIN PLAN SET statement_id = 'john' for
  2  SELECT COUNT(*) FROM demographic
  3  WHERE patient_first_name = 'JOHN';
Explained.
SQL> SELECT plan_table_output
  2  FROM TABLE(dbms_xplan.display('plan_table','john','serial'));
PLAN_TABLE_OUTPUT
| Id  | Operation            |  Name        | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT     |              |     1 |    17 |  1969 |
|   1 |  SORT AGGREGATE      |              |     1 |    17 |       |
|*  2 |   TABLE ACCESS FULL  | DEMOGRAPHIC  |  1547 | 26299 |  1969 |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - filter("DEMOGRAPHIC"."PATIENT_FIRST_NAME"='JOHN')
Note: cpu costing is offNote that explain plan reports that it used the predicate as a filter during the full scan. Now, 1 = 1:
SQL> EXPLAIN PLAN SET statement_id = 'johna' for
  2  SELECT COUNT(*) FROM demographic
  3  WHERE 1 = 1;
Explained.
SQL> SELECT plan_table_output
  2  FROM TABLE(dbms_xplan.display('plan_table','johna','serial'));
PLAN_TABLE_OUTPUT
| Id  | Operation             |  Name          | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT      |                |     1 |       |   119 |
|   1 |  SORT AGGREGATE       |                |     1 |       |       |
|   2 |   INDEX FAST FULL SCAN| DEMOGRAPHIC_4  |   154K|       |   119 |
Note: cpu costing is offNote that there is no predicate used. Even combining both, I still get only one predicate evaluated:
SQL> EXPLAIN PLAN SET statement_id = 'johnb' for
  2  SELECT COUNT(*) FROM demographic
  3  WHERE patient_first_name = 'JOHN' and
  4        1 = 1;
Explained.
SQL> SELECT plan_table_output
  2  FROM TABLE(dbms_xplan.display('plan_table','johnb','serial'));
PLAN_TABLE_OUTPUT
| Id  | Operation            |  Name        | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT     |              |     1 |    17 |  1969 |
|   1 |  SORT AGGREGATE      |              |     1 |    17 |       |
|*  2 |   TABLE ACCESS FULL  | DEMOGRAPHIC  |  1547 | 26299 |  1969 |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - filter("DEMOGRAPHIC"."PATIENT_FIRST_NAME"='JOHN')
Note: cpu costing is offJohn