PL/SQL Variables and Program Data: using different data types

PL/SQL programs normally are used to manipulate database information. You commonly do this by declaring variables and data structures in your programs, and then working with that PL/SQL-specific data.

A variable is a named instantiation of a data structure declared in a PL/SQL block (either locally or in a package). Unless you declare a variable as a CONSTANT, its value can be changed at any time in your program.

The following table summarizes the different types of program data:

PL/SQL Scalar Datatypes

Scalar datatypes divide into four families: number, character, datetime, and Boolean. Subtypes further define a base datatype by restricting the values or size of the base datatype.

Numeric datatypes

Numeric datatypes represent real numbers, integers, and floating-point numbers. They are stored as NUMBER, PLS_INTEGER, and IEEE floating-point storage types.

Decimal numeric datatypes store fixed and floating-point numbers of just about any size. They include the subtypes NUMBER, DEC, DECIMAL, NUMERIC, FLOAT, REAL, and DOUBLE PRECISION. The maximum precision of a variable with type NUMBER is 38 digits, which yields a range of values from 1.0E-129 through 9.999E125.

Variables of type NUMBER can be declared with precision and scale, as follows:

NUMBER(precision, scale)

where precision is the number of digits, and scale is the number of digits to the right (positive scale) or left (negative scale) of the decimal point at which rounding occurs. Legal values for scale range from −84 to 127. The following table shows examples of precision and scale:

Oracle provides a variety of datatypes to store whole numbers: BINARY_INTEGER, INTEGER, INT, SMALLINT, NATURAL, NATURALN, POSITIVE, POSITIVEN, SIGNTYPE, and PLS_INTEGER.

Binary integer datatypes store signed integers in the range of −2³¹ + 1 to 2³¹ − 1. The subtypes include NATURAL (0 through 2³¹ − 1) and POSITIVE (1 through 2³¹ − 1) together with the NOT NULL variations NATURALN and POSITIVEN. SIGNTYPE is restricted to three values (−1, 0, 1). PLS_INTEGER is an unconstrained subtype (alias) of BINARY_INTEGER.

SIMPLE_INTEGER has the same range as BINARY_INTEGER except that it does not allow for null values and does not raise an exception if an overflow occurs. For example, 2147483647 + 1 = −2147483648 (note the negative value!). SIMPLE_INTEGER datatypes can result in significantly faster execution speeds when the PL/SQL code is compiled to native machine code.

IEEE 754–compliant floating-point numbers are available in both SQL and PL/SQL. These subtypes are the single-precision BINARY_FLOAT and the double-precision BINARY_DOUBLE. Because these datatypes require less memory and use native machine arithmetic, they perform much better for scientific or engineering applications that are computer-intensive or that require comparison to infinity or not a number (NaN). These two datatypes have binary precision instead of the decimal precision used in the NUMBER family. So, if you are developing financial applications that are concerned with rounding errors or require decimal precision, you probably should not use these floating-point datatypes.

The following table lists the PL/SQL numeric datatypes with ANSI and IBM compatibility. In this table:

• precision is the precision for the subtype.
• scale is the scale of the subtype.
• binary is the binary precision of the subtype.

Character datatypes

Character datatypes store alphanumeric text and are manipulated by character functions. As with the numeric family, there are several subtypes in the character family, shown in the following table:

Unicode character datatypes

The standard WE8MSWIN1252 or WE8ISO8859P2 character set does not support some languages, such as Chinese and Greek. To support multiple languages, the database allows two character sets—the database character set and a Unicode character set, sometimes called the national character set (NLS).

The two NLS datatypes, NCHAR and NVARCHAR2, are used to represent data in the Unicode character set. NCHAR values are fixed-length character data; the maximum length is 32,767 bytes. NVARCHAR2 values are variable-length character data; the maximum length also is 32,767 bytes.

Datetime datatypes

The datetime datatypes are DATE, TIMESTAMP, TIMESTAMP WITH TIME ZONE, and TIMESTAMP WITH LOCAL TIME ZONE. There are also two interval datatypes, INTERVAL YEAR TO MONTH and INTERVAL DAY TO SECOND.

DATE values are fixed-length, date-plus-time values. The DATE datatype can store dates from January 1, 4712 BC to December 31, 9999 AD. Each DATE includes the century, year, month, day, hour, minute, and second. Subsecond granularity is not supported via the DATE datatype; use one of the TIMESTAMP datatypes instead. The time portion of a DATE defaults to midnight (12:00:00 a.m.) if it is not included explicitly.

TIMESTAMP values store date and time to subsecond granularity. The subsecond precision (the number of digits to the right of the decimal) either defaults to 6 or is set to 0 through 9 digits by declaration, as in:

DECLARE 
   mytime_declared TIMESTAMP(9); -- max precision 
   mytime_default TIMESTAMP; -- default 6 digits precision 
   mytime_to_seconds TIMESTAMP(0); -- no fractional seconds

TIMESTAMP WITH TIME ZONE values store date and time values like a TIMESTAMP but also store the hourly offset from Coordinated Universal Time (UTC, which is essentially equivalent to Greenwich Mean Time). As with TIMESTAMP, the subsecond precision is 0 to 9 digits, either declared or inherited from the default 6 digits of precision:

DECLARE  
    mytime_declared TIMESTAMP(9) WITH TIME ZONE;  
    mytime_default  TIMESTAMP WITH TIME ZONE;

TIMESTAMP WITH LOCAL TIME ZONE values store date and time values together with the UTC offset, like a TIMESTAMP WITH TIME ZONE. The principal difference between these timestamp datatypes occurs when values are saved to or retrieved from a database table. TIMESTAMP WITH LOCAL TIME ZONE values are converted to the database time zone and saved without an offset. The values retrieved from the database table are converted from the database time zone to the session’s time zone.

The offset from UTC for both TIMESTAMP WITH TIME ZONE and TIMESTAMP WITH LOCAL TIME ZONE can be hours and minutes or a time zone region (found in the V$TIMEZONE_NAMES data dictionary view) with the optional daylight savings time name (also found in V$TIMEZONE_NAMES). For example:

ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT=   
       'DD-Mon-YYYY HH24:MI:SS.FF TZR'; 
DECLARE   
       my_tswtz    TIMESTAMP(4) WITH TIME ZONE;BEGIN   
       my_tswtz := '31-MAR-2016 07:32:45.1234 US/Pacific';

INTERVAL YEAR TO MONTH values store a period of time in years and months:

DECLARE  
      myy2m INTERVAL YEAR TO MONTH;
BEGIN  
      myy2m := INTERVAL '1–6' YEAR TO MONTH; -- interval of 18 months

INTERVAL DAY TO SECOND values store a period of time in days, hours, minutes, seconds, and fractional seconds:

DECLARE  
      myd2s INTERVAL DAY TO SECOND;
BEGIN  
      myd2s := INTERVAL '2 10:32:15.678' DAY TO SECOND;

BOOLEAN datatype

The BOOLEAN datatype can store one of three values: TRUE, FALSE, or NULL. BOOLEAN variables usually are used in logical control structures such as IF . . . THEN or LOOP statements.

The following truth tables show the results of logical AND, OR, and NOT operations with PL/SQL’s three-value Boolean model:

LOB Datatypes

PL/SQL supports a number of large object (LOB) datatypes, which can store objects up to 8 terabytes in size, or even up to 128 TB, depending on database block size. Unlike the scalar datatypes, variables declared for LOBs use locators, or pointers to the actual data. LOBs are manipulated in PL/SQL using the built-in package DBMS_LOB. The LOB datatypes are:

BFILE

File locators pointing to read-only large binary objects in operating system files. With BFILEs, the large objects are outside the database, and the maximum size is system-dependent.

BLOB

LOB locators that point to large binary objects inside the database.

CLOB

LOB locators that point to large character (alphanumeric) objects inside the database.

NCLOB

LOB locators that point to large Unicode character objects inside the database.

Implicit Datatype Conversions

Whenever PL/SQL detects that a datatype conversion is necessary, it attempts to change the values as required to perform the operation. Not all values in each datatype can be converted to another datatype. For example, an attempt to convert BINARY_FLOAT_NAN to a number datatype will raise an INVALID NUMBER exception.

NULLs in PL/SQL

PL/SQL represents unknown or inapplicable values as NULL values. Because a NULL is unknown, a NULL is never equal or not equal to anything (including another NULL value). In addition, most functions and operators return a NULL when passed a NULL argument. You cannot check for equality or inequality to NULL; therefore, you must use the IS NULL or IS NOT NULL syntax to check for NULL values.

Here is an example of the IS NULL syntax used to check the value of a variable:

BEGIN

  /* Correct usage */
   IF myvar IS NULL THEN ...

  /* Dangerous! Always evaluates to NULL */
  IF myvar = NULL THEN ...

There are several ways in SQL and PL/SQL to substitute a non-null value for a null variable or expression. NVL is commonly used:

NVL (exprn1, exprn2)

With NVL, Oracle will always evaluate both arguments, even if exprn1 is not null. A potentially more efficient function is COALESCE:

COALESCE(exprn1, exprn2 [, exprn3, ...] )

COALESCE has two distinct characteristics: first, it accepts more than two arguments; second, it uses short-circuit evaluation, meaning it stops looking for a non-null value when it finds one (and, therefore, does not evaluate every expression in the list, which NVL does).

Declaring Variables

Before you can use a variable, you must first declare it in the declaration section of your PL/SQL block or in a package as a global. When you declare a variable, PL/SQL allocates memory for the variable’s value and names the storage location so that the value can be retrieved and changed. The syntax for a variable declaration is:

variable_name [CONSTANT] datatype [NOT NULL]
    [{ := | DEFAULT } initial_value];

Constrained declarations

A programmer may constrain certain datatypes at declaration so that variables have a size, scale, or precision that is less than the maximum allowed. Some examples:

total_sales              NUMBER(15,2); -- Constrained
emp_id                   VARCHAR2(9); -- Constrained
company_number    NUMBER; -- Unconstrained
book_title               VARCHAR2; -- Not valid

Constrained declarations can require less memory than unconstrained declarations. Not all datatypes can be declared unconstrained; VARCHAR2 is a notable exception.

Constants

The CONSTANT keyword in a declaration requires an initial value and does not allow that value to be changed in the program. For example:

min_order_qty   NUMBER(1) CONSTANT := 5;

Default values

The PL/SQL runtime engine assigns a default value of NULL to each variable you explicitly declare.¹ If you want to initialize a variable to a value other than NULL, you can do so in the declaration with either the assignment operator (:=) or the DEFAULT keyword:

counter     BINARY_INTEGER := 0;
priority     VARCHAR2(8) DEFAULT 'LOW';

A NOT NULL constraint can be appended to the variable’s datatype declaration to indicate that NULL is not a valid value. If you add the NOT NULL constraint, you must explicitly assign an initial value for that variable.

Anchored Declarations

Use the %TYPE attribute to anchor the datatype of a scalar variable to either another variable or to a column in a database table or view. Use %ROWTYPE to anchor a record’s declaration to a cursor or table.

The following block shows several variations of anchored declarations:

DECLARE
   tot_sales NUMBER(20,2);

  -- Anchor to a local variable
  monthly_sales tot_sales%TYPE;

  -- Anchor to a database column
  v_ename employee.last_name%TYPE;

  -- Anchor to a cursor
  CURSOR mycur IS SELECT * FROM employee;
  myrec mycur%ROWTYPE;

         -

The NOT NULL clause on a variable declaration (but not on a database column definition) follows the %TYPE anchoring and requires anchored declarations to have a default in their declaration. The default value for an anchored declaration can be different from that for the base declaration:

tot_sales            NUMBER(20,2) NOT NULL DEFAULT 0;
monthly_sales    tot_sales%TYPE DEFAULT 10;

Programmer-Defined Subtypes

PL/SQL allows you to define unconstrained scalar subtypes, which you might want to do for greater “self-documentation” of your code or for future-proofing. An unconstrained subtype provides an alias to the original underlying datatype; for example:

CREATE OR REPLACE PACKAGE std_types
IS
   -- Declare reusable app-specific type
   SUBTYPE dollar_amt_t IS NUMBER; 
END std_types;

CREATE OR REPLACE PROCEDURE process_money
IS
   -- Use standardized type declared in package
   credit std_types.dollar_amt_t;

A constrained subtype limits or constrains the new datatype to a subset of the original datatype. For example, POSITIVE is a constrained subtype of BINARY_INTEGER. The declaration for POSITIVE in the STANDARD package is:

SUBTYPE POSITIVE IS BINARY_INTEGER RANGE 1..2147483647;

You can define your own constrained subtypes in your programs:

PACKAGE std_types
IS
    SUBTYPE currency_t IS NUMBER (15, 2);
END;

Вас заинтересует / Intresting for you:

PL/SQL package: Collecting Rel... 1388 views sepia Sat, 01 Dec 2018, 10:54:57

Describe the Basic PL/SQL Prog... 1739 views Андрей Волков Wed, 12 Sep 2018, 15:23:39

Why Oracle DBAs learn PL/SQL a... 1651 views Андрей Волков Wed, 12 Sep 2018, 14:43:12

Translating PL/SQL to JavaScri... 3332 views Гвен Sun, 03 Jun 2018, 11:54:40