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Introduction To SQL
Structured Query Language (sql) is the set of statements with which all programs and users access data in an Oracle database. Application programs and Oracle tools often allow users access to the database without using sql directly, but these applications in turn must use sql when executing the user's request. This chapter provides background information on sql as used by most database systems.
Each value manipulated by Oracle has a datatype. A value's datatype associates a fixed set of properties with the value. These properties cause Oracle to treat values of one datatype differently from values of another. For example, you can add values of NUMBER datatype, but not values of RAW datatype.
The terms literal and constant value are synonymous and refer to a fixed data value. For example, 'JACK', 'BLUE ISLAND', and '101' are all character literals; 5001 is a numeric literal. Character literals are enclosed in single quotation marks, which enable Oracle to distinguish them from schema object names.Read on to know more about SQL Database .
If a column in a row has no value, then the column is said to be null, or to contain a null. Nulls can appear in columns of any datatype that are not restricted by NOT NULL or PRIMARY KEY integrity constraints. Use a null when the actual value is not known or when a value would not be meaningful.
A pseudocolumn behaves like a table column, but is not actually stored in the table. You can select from pseudocolumns, but you cannot insert, update, or delete their values. We will discuss CURRVAL and NEXTVAL, LEVEL, ROWID, ROWNUM.Read on to know more about SQL Database
Comments within SQL statements do not affect the statement execution, but they may make your application easier for you to read and maintain. You may want to include a comment in a statement that describes the statement's purpose within your application.
Datatbase Objects
There are two types of Objects Schema Object and Non Schema Objects. A schema is a collection of logical structures of data, or schema objects.Read on SQL Tutorial to know more.
Schema Object Names and Qualifiers
This section provides:Rules for naming schema objects and schema object location qualifiers, Guidelines for naming schema objects and qualifiers
Queries and Subqueries
A query is an operation that retrieves data from one or more tables or views. In this reference, a top-level SELECT statement is called a query, and a query nested within another SQL statement is called a subquery.
Types of SQL Statements
The CREATE, ALTER, and DROP commands require exclusive access to the specified object. For example, an ALTER TABLE statement fails if another user has an open transaction on the specified table.Read on to know more about Oracle DBA
Savepoint names must be distinct within a given transaction. If you create a second savepoint with the same identifier as an earlier savepoint, the earlier savepoint is erased. After a savepoint has been created, you can either continue processing, commit your work, roll back the entire transaction, or roll back to the savepoint. Read on to know about Oracle DBA.

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