You use identifiers to name the PL/SQL program items and units that include constants, variables, cursors, exceptions, cursor variables, subprograms, and packages. Some of the examples of identifiers is shown below:
An identifier consists of a letter optionally followed by many letters, numerals, underscores, dollar signs, and number signs. Other characters like slashes, hyphens, and spaces are illegal, as the examples shown below:
mine&yours -- illegal ampersand
debit-amount -- illegal hyphen
on/off -- illegal slash
user id -- illegal space
The next examples represents that adjoining and trailing dollar signs, underscores, and number signs are permitted:
You can use lower, upper, or mixed case to write the identifiers. The PL/SQL is not case sensitive except within the string and character literals. Therefore, if the only difference between identifiers is the case of corresponding letters, then PL/SQL considers the identifiers to be similar, as the example shown below:
LastName-- same as lastname
LASTNAME-- same as lastname and LastName
The length of an identifier may not exceed 30 characters. But, each character, involving underscores, dollar signs, and number signs, is significant. For example, the PL/SQL considers the following identifiers to be different:
Identifiers must be descriptive. And hence, avoid obscure names like cpm. Rather, use of meaningful names like cost_per_thousand.
The Identifiers worldwide declared in package STANDARD, like the exception INVALID_NUMBER, can be re-declared. Though, re-declaring predefined identifiers is error prone as your local declaration overrides the global declaration.
For flexibility, the PL/SQL encloses identifiers within the double quotes. The Quoted identifiers are seldom required, but rarely can they be useful. They can contain any sequence of printable characters together with spaces but excluding the double quotes. And hence, the following identifiers are valid:
"*** header info ***"
The highest length of a quoted identifier is 30 characters not counting the double quotes. However allowed, using the PL/SQL reserved words as quoted identifiers is a poor programming practice.
Some of the PL/SQL reserved words are not reserved by the SQL. For example, you can use the PL/SQL reserved word TYPE in a CREATE TABLE statement to name a database column. But, if a SQL statement in your program refers to that column, you get a compilation error, as the following example is shown below:
SELECT acct, type, bal INTO ...-- causes compilation error
To prevent the error, enclose the uppercase column name in double quotes, as shown below:
SELECT acct, "TYPE", bal INTO ...
The column name cannot appear in the lower or mixed case (unless it was defined that way in the CREATE TABLE statement). For example, the statement below is invalid:
SELECT acct, "type", bal INTO ...-- causes compilation error
Otherwise, you can create a view that renames the troublesome column, then use the view rather of the base table in SQL statements.