------------------ SQL (Oracle applied)

What are DML and DDL

DDL

Data definition language (DDL) statements define, structurally change, and

drop schema objects.

An implicit COMMIT occurs immediately before the database executes a DDL

statement and a COMMIT or ROLLBACK occurs immediately afterward.

DML

Data manipulation language (DML) statements query or manipulate data in

existing schema objects. Whereas DDL statements enable you to change the

structure of the database, DML statements enable you to query or change the

contents. For example, ALTER TABLE changes the structure of a table, whereas

INSERT adds one or more rows to the table.

Agregate functions with examples

todo

What is a nested subquery?

A subquery is a SELECT statement nested within another SQL statement. Subqueries

are useful when you must execute multiple queries to solve a single problem.

These subqueries can reside in the WHERE clause, the FROM clause, or the SELECT clause.

Example:

SELECT first_name, last_name

FROM employees

WHERE department_id

IN (SELECT department_id FROM departments WHERE location_id = 1800);

Types of indices in Oracle

Oracle Database provides several indexing schemes, which provide complementary

performance functionality. The indexes can be categorized as follows:

B-tree indexes

These indexes are the standard index type. They are excellent for primary

key and highly-selective indexes. Used as concatenated indexes, B-tree

indexes can retrieve data sorted by the indexed columns.

B-tree indexes have the following subtypes:

� Index-organized tables

An index-organized table differs from a heap-organized because the data is

itself the index. See "Overview of Index-Organized Tables" on page 3-20.

� Reverse key indexes

In this type of index, the bytes of the index key are reversed, for

example, 103 is stored as 301. The reversal of bytes spreads out inserts

into the index over many blocks.

� Descending indexes

This type of index stores data on a particular column or columns in

descending order.

� B-tree cluster indexes

This type of index is used to index a table cluster key. Instead of

pointing to a row, the key points to the block that contains rows related

to the cluster key.

Bitmap and bitmap join indexes

In a bitmap index, an index entry uses a bitmap to point to multiple rows.

In contrast, a B-tree index entry points to a single row. A bitmap join

index is a bitmap index for the join of two or more tables.

Function-based indexes

This type of index includes columns that are either transformed by a

function, such as the UPPER function, or included in an expression.

B-tree or bitmap indexes can be function-based.

Application domain indexes

This type of index is created by a user for data in an application-specific

domain. The physical index need not use a traditional index structure and

can be stored either in the Oracle database as tables or externally as a

file.

What is a hierarchical query? How to create it?

http://docs.oracle.com/cd/B19306_01/server.102/b14200/queries003.htm

If a table contains hierarchical data, then you can select rows in a hierarchical order using the hierarchical query clause:

[ START WITH condition ] CONNECT BY [ NOCYCLE ] condition

Example:

1)show EXPLAIN PLAN

-- EXPLAIN PLAN

delete PLAN_TABLE where statement_id = 'ago01';

EXPLAIN PLAN SET STATEMENT_ID = 'ago01' FOR

---------------------- put query below

select /*+ FIRST_ROWS(25) */

decode(message_type, '1','personal', '2','broadcast') as mess_type,

eb_messages_data.*

from eb_messages_data

where message_id in (select message_id

from eb_disposer_mailboxes

where disposer_number like '%3140007')

and message_type = '2';

----------------------- put query above

select * from PLAN_TABLE where STATEMENT_ID = 'ago01';

SELECT lpad('.',level-1, '.')||operation||' '|| options||' '||object_type||' '||object_name "Plan",

cost, bytes, cardinality, access_predicates || filter_predicates as predicates

FROM PLAN_TABLE

CONNECT BY prior id = parent_id

AND prior statement_id = statement_id

START WITH id = 0

AND statement_id = 'ago01'

ORDER BY id;

2)CONNECT BY Example

SELECT employee_id, last_name, manager_id

FROM employees

CONNECT BY PRIOR employee_id = manager_id;

3)level example

SELECT employee_id, last_name, manager_id, LEVEL

FROM employees

CONNECT BY PRIOR employee_id = manager_id;

4)START WITH Examples

SELECT last_name, employee_id, manager_id, LEVEL

FROM employees

START WITH employee_id = 100

CONNECT BY PRIOR employee_id = manager_id

ORDER SIBLINGS BY last_name;

What is query tuning? How to tune a query?

http://www.dba-oracle.com/art_sql_tune.htm

http://www.akadia.com/services/ora_query_tuning.html

http://www.orafaq.com/wiki/Oracle_database_Performance_Tuning_FAQ

Partitioning and methods of partitioning

Partitioning enables you to decompose very large tables and indexes into

smaller and more manageable pieces called partitions. Each partition is an

independent object with its own name and optionally its own storage characteristics.

From the perspective of an application, only one schema object exists. DML statements require no modification

to access partitioned tables. Partitioning is useful for many different types of database applications, particularly

those that manage large volumes of data. Benefits include:

Increased availability

Easier administration of schema objects

Reduced contention for shared resources in OLTP systems

Enhanced query performance in data warehouses

Partitioning Strategies:

Range Partitioning

CREATE TABLE time_range_sales

( prod_id NUMBER(6)

, cust_id NUMBER

, time_id DATE

, channel_id CHAR(1)

, promo_id NUMBER(6)

, quantity_sold NUMBER(3)

, amount_sold NUMBER(10,2)

)

PARTITION BY RANGE (time_id)

(PARTITION SALES_1998 VALUES LESS THAN (TO_DATE('01-JAN-1999','DD-MON-YYYY')),

PARTITION SALES_1999 VALUES LESS THAN (TO_DATE('01-JAN-2000','DD-MON-YYYY')),

PARTITION SALES_2000 VALUES LESS THAN (TO_DATE('01-JAN-2001','DD-MON-YYYY')),

PARTITION SALES_2001 VALUES LESS THAN (MAXVALUE)

);

List Partitioning

CREATE TABLE list_sales

( prod_id NUMBER(6)

, cust_id NUMBER

, time_id DATE

, channel_id CHAR(1)

, promo_id NUMBER(6)

, quantity_sold NUMBER(3)

, amount_sold NUMBER(10,2)

)

PARTITION BY LIST (channel_id)

(PARTITION even_channels VALUES (2,4),

PARTITION odd_channels VALUES (3,9)

);

Hash Partitioning

CREATE TABLE hash_sales

( prod_id NUMBER(6)

, cust_id NUMBER

, time_id DATE

, channel_id CHAR(1)

, promo_id NUMBER(6)

, quantity_sold NUMBER(3)

, amount_sold NUMBER(10,2)

)

PARTITION BY HASH (prod_id)

PARTITIONS 2;

------------------ DB

Primary key vs unique key. Differences.

What types of constraints does one know?

NOT NULL

Allows or disallows inserts or updates of rows

containing a null in a specified column.

Unique key

Prohibits multiple rows from having the same value in

the same column or combination of columns but allows

some values to be null.

Primary key

Combines a NOT NULL constraint and a unique constraint. It prohibits

multiple rows from having the same value in the same column or combination

of columns and prohibits values from being null

Foreign key

Designates a column as the foreign key and establishes a relationship

between the foreign key and a primary or unique key, called the referenced

key.

Check

Requires a database value to obey a specified condition.

REF

Dictates types of data manipulation allowed on values in a REF column and

how these actions affect dependent values. In an object-relational database,

a built-in data type called a REF encapsulates a reference to a row object

of a specified object type. Referential integrity constraints on REF columns

ensure that there is a row object for the REF.

What is a view/materialized view.

View

A view is a logical representation of one or more tables. In essence,

a view is a stored query. A view derives its data from the tables on which

it is based, called base tables. Base tables can be tables or other views.

All operations performed on a view actually affect the base tables. You can

use views in most places where tables are used.

Materialized View

Materialized views are query results that have been stored or "materialized"

in advance as schema objects. The FROM clause of the query can name tables,

views, and materialized views. Collectively these objects are called master

tables (a replication term) or detail tables (a data warehousing term).

What kind of joins do you know?

Inner joins

An inner join is a join of two or more tables that returns only rows that

satisfy the join condition. For example, if the join condition is employees.

department_id=departments.department_id, then rows that do not satisfy this

condition are not returned.

Outer joins

An outer join returns all rows that satisfy the join condition and also

returns rows from one table for which no rows from the other table satisfy

the condition. For example, a left outer join of employees and departments

retrieves all rows in the employees table even if there is no match in

departments. A right outer join retrieves all rows in departments even if

there is no match in employees.

Cartesian products

If two tables in a join query have no join condition, then the database

returns their Cartesian product. Each row of one table combines with each

row of the other. For example, if employees has 107 rows and departments

has 27, then the Cartesian product contains 107*27 rows. A Cartesian

product is rarely useful.

Types of tables (regular, temporary, index-organized etc)

Oracle Database tables fall into the following basic categories:

Relational tables: Relational tables have simple columns and are the most common table type.

A heap-organized table

does not store rows in any particular order. The CREATE

TABLE statement creates a heap-organized table by default.

An index-organized table

orders rows according to the primary key values. For

some applications, index-organized tables enhance performance and use disk

space more efficiently.

An external table

is a read-only table whose metadata is stored in the database but

whose data in stored outside the database.

Object tables: The columns correspond to the top-level attributes of an object type.

--

A table is either permanent or temporary. A permanent table definition and data

persist across sessions. A temporary table definition persists in the same way as a

permanent table definition, but the data exists only for the duration of a transaction or

session. Temporary tables are useful in applications where a result set must be held

temporarily, perhaps because the result is constructed by running multiple operations.

Are database Indexes useful? What is the role of them?

An index is an optional structure, associated with a table or table cluster,

that can sometimes speed data access. By creating an index on one or more

columns of a table, you gain the ability in some cases to retrieve a small

set of randomly distributed rows from the table. Indexes are one of many means

of reducing disk I/O.

If a heap-organized table has no indexes, then the database must perform a

full table scan to find a value. For example, without an index, a query of

location 2700 in the hr.departments table requires the database to search

every row in every table block for this value. This approach does not scale

well as data volumes increase.

Why many indexes are not good for performance

todo

What does it mean database de-normalization?

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Which of SELECT, UPDATE, DELETE, ADD�s performance is mostly affected by performance of indexes?

todo

What are ways to increase performance of database

todo