How to use declare in oracle

The DECLARE statement in Oracle declares objects such as variables, constants, and cursors. The syntax is: DECLARE [modifier] identifier data type [DEFAULT default value] [, ...], used to store temporary data, multiple reference values, or process cursors.

DECLARE Usage in Oracle

The DECLARE statement is used to declare variables and constants in Oracle PL/SQL blocks , cursors, and other objects. The syntax is as follows:

<code>DECLARE [修饰符] 标识符 数据类型 [DEFAULT 默认值] [, ...]</code>

Among them:

• Modifier: Optional, specifies the visibility of the variable (LOCAL/GLOBAL).
• Identifier: The name of the variable or constant.
• Data type: The type of variable or constant (for example: NUMBER, VARCHAR2).
• DEFAULT: Optional, specifies the default value of the variable.

Example:

DECLARE
  v_number NUMBER(10);
  c_name VARCHAR2(50) := 'John Doe';
  r_cursor SYS_REFCURSOR;
BEGIN
  ...
END;

In the above example:

• v_number is a LOCAL variable used to store a 10 digits.
• c_name is a global constant used to store the string "John Doe".
• r_cursor is a cursor variable used to reference the result set of a SELECT statement.

Visibility of variables and constants:

• LOCAL: A variable can only be used within the block in which it is declared.
• GLOBAL: A variable can be used within the block in which it is declared and its nested blocks.

When to use DECLARE?

The DECLARE statement is usually used in the following situations:

• It is necessary to store temporary data (variables) in a PL/SQL block.
• Need to reference the same value (constant) multiple times in a block.
• Need to handle cursors in blocks.

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