How do you use Connect by prior?
How do you use Connect by prior?
The PRIOR operator to refer to the parent row can be used as following: Sample query on the employees table: SELECT employee_id, last_name, manager_id, LEVEL FROM employees CONNECT BY PRIOR employee_id = manager_id; shows employee_id, last_name, manager_id and level in the tree for the employee hierarchy.
What is dual level?
Level is used to count the depth of the connections, so a parent would be 1, a child 2, a child of the child 3, etc. So think of it a rownum for hierarchical queries. https://stackoverflow.com/questions/42184715/select-level-from-dual-connect-by-level-4-how-it-works-internally/42184836#42184836.
What is connect by?
CONNECT BY specifies the relationship between parent rows and child rows of the hierarchy. The NOCYCLE parameter instructs Oracle Database to return rows from a query even if a CONNECT BY LOOP exists in the data.
What is connect by level?
The term LEVEL refers to a Pseudocolumn in Oracle which is used in a hierarchical query to identify the hierarchy level (parent->child) in numeric format. The CONNECT BY clause defines the hierarchical relationship between the parent rows and the child rows of the hierarchy.
What can be present in the body of forall?
What can be present in the body of a FORALL statement?
- One or more DML statements.
- A single DML statement.
- A single SELECT or DML statement.
- One or more SELECT statement.
How Tkprof will help to increase the performance?
The SORT value causes TKPROF to sort the SQL statements in order of the sum of the CPU time spent executing and the CPU time spent fetching rows before writing them to the output file. For greatest efficiency, always use SORT parameters.
What is difference between for and forall in Oracle?
The FORALL allows to perform the DML operations on data in bulk. It is similar to that of FOR loop statement except in FOR loop things happen at the record-level whereas in FORALL there is no LOOP concept. Instead the entire data present in the given range is processed at the same time.
Where can a subprogram be created?
You can create a subprogram either inside a PL/SQL block (which can be another subprogram), inside a package, or at schema level. A subprogram created inside a PL/SQL block is a nested subprogram.
What is Tkprof and how is it used?
TKPROF accepts as input a trace file produced by the SQL Trace facility, and it produces a formatted output file. TKPROF can also be used to generate execution plans.
How do I run Tkprof?
Oracle : TKPROF simple steps
- Set the following parameters. TIMED_STATISTICS = TRUE.
- Enable SQL TRACE for a session. alter session set SQL_TRACE true;
- Run the query.
- obtain the number included in the trace filename.
- Run TKPROF at the command line to put the TRACE file into readable format.
When should I use bulk collect?
Use the BULK COLLECT clause to fetch multiple rows into one or more collections with a single context switch. Use the FORALL statement when you need to execute the same DML statement repeatedly for different bind variable values.
Why we use bulk collect in Oracle?
Oracle PL/SQL provides the functionality of fetching the records in bulk rather than fetching one-by-one. The main advantage of using BULK COLLECT is it increases the performance by reducing the interaction between database and PL/SQL engine.
What does level mean in connectconnect by?
CONNECT BY queries allow some pseudo-columns. One of those pseudo-columns is LEVEL, which indicates the current level of the hierarchy (where level 1 represents the top of the hierarchy). The projection clause of the query can use LEVEL as a column.
How to use the (start with) CONNECT BY clause?
This page explains the hierarchical query using the (start with) connect by clause in an Oracle SQL Query. The START WITH clause is optional and specifies the rows athat are the root (s) of the hierarchical query. If you omit this clause, then Oracle uses all rows in the table as root rows. The START WITH condition can contain a subquery,
What is the difference between levellevel and dual?
LEVEL must be used with CONNECT BY Clause. The CONNECT BY clause defines the hierarchical relationship between the parent rows and the child rows of the hierarchy. DUAL is a dummy table automatically generated by Oracle database along with data dictionary. Above query will execute Level having initial value 1 from dual, which is dummy table.
What are the restrictions of connect_by_condition?
The connect_by_condition can be any condition, however, it must use the PRIOR operator to refer to the parent row. Restriction on the CONNECT BY clause: The connect_by_condition cannot contain a regular subquery or a scalar subquery expression.
