What is the difference between executing query statements using 'execute' and directy in PLSQL?

Question

st :='SELECT USERNAME FROM LOGIN WHERE USERNAME =: a and PASSWORD =: b';
execute immediate st into un using username,pw;

Or

SELECT USERNAME INTO un FROM LOGIN WHERE USERNAME = username and PASSWORD = pw;

where username, un and pw are varchar2(50) variables

Answer 1

Every PL/SQL will be first compiled before execution. Compilation here involves, both syntax checks of PL/SQL constructs and semantic checks over the SQLs being used.

Running SQLs via EXECUTE IMMEDIATE means, the query is parsed and executed dynamically. And then we name it Dynamic SQL.The dynamic SQLs are formed using a string, and the query , hence is not known to Oracle before hand. And only during runtime, it is compiled and executed.

Whereas, in the other method, the SQL is parsed for semantic checks during compilation itself. We call it as Static SQL. We get,all the invalid SQL statement errors during compilation itself.

The other main difference, would be, when a Static SQL is been used in a PROCEDURE or FUNCTION. Oracle would create a dependency of that table name with the Procedure/function. You can find them in the dba_dependencies table.

It is very important to create such dependencies because, when the table is being altered with modifying the column name that is referred in a function or the table/view/type itself is dropped, Then Oracle would INVALIDATE the function. Thus making sure, we dont get unexpected Exceptions.But DYNAMIC sqls would be completely anonymous and have no dependencies created. You may get a ORA-942 TABLE OR VIEW DOESNOT EXISTS error, only with DYNAMIC sql and not with a Static SQL, just because of this reason.

The below query would list out all the table names referred in the procedure or function, either as a Synonym(table referred cross schemas, without schema name) and tablename directly with schema name(or default schema)

select 
   proc_syn.referenced_owner, 
   proc_syn.referenced_name, 
   proc_syn.referenced_type,
   syn_tab.table_name
from 
   dba_dependencies proc_syn, dba_synonyms syn_tab, dba_tables tables
where 
     proc_syn.name= 'YOUR_PROC' 
  AND REFERENCED_TYPE in ( 'SYNONYM','TABLE')
  AND proc_syn.referenced_name = syn_tab.synonym_name
  AND syn_tab.synonym_name = tables.table_name
  AND syn_tab.owner = 'PUBLIC'
order by 
  proc_syn.referenced_owner, syn_tab.table_name;

Answer 2

Execute immediate is used to invoke dynamic SQL for more information see oracle Documents