Discuss the meaning, implementation and application of equality in Oracle

Oracle is one of the most famous relational database management systems in the world, which is widely used in enterprise-level applications. Because of its reliability, stability and high performance, Oracle has become the database of choice for large enterprises. In Oracle, equality is an important concept. This article will explore the meaning, implementation and application of equality in Oracle.

1. The meaning of equality

In Oracle, equality indicates whether two values ​​are equal. Values ​​can be numbers, characters, dates, Boolean, etc. In the physical world, two objects are equal only if they are equal in all aspects. Likewise, in Oracle, two values ​​are equal only if they are equal in all respects. Therefore, the meaning of equality depends on the data type. The equivalent meanings of numbers, characters, dates, and Boolean types are explained below.

1. Equality of numeric types

In Oracle, equality of numeric types is strict. If two numbers have the same value, they are considered equal. For example, 3 and 3 are equal, but 3 and 3.0 are not equal because they are different data types.

2. Equality of character types

The definition of equality of character types is also strict. Two characters are considered equal if they have the same literal value and the same case. For example, 'ABC' and 'ABC' are equal, but 'ABC' and 'abc' are not.

3. Equality of date types

In Oracle, equality of date types is exact. Two dates are considered equal only if they are exactly the same. For example, two dates are equal if they have the same year, month, and day.

4. Boolean type equality

In Oracle, there are only two possibilities for Boolean type equality: True or False. If two Boolean values ​​are equal, they have the same Boolean state. If they are different, they have different boolean states.

2. Implementation of equality

The method of achieving equality in Oracle is different from other relational database management systems. Oracle implements equality using Oracle's internal mechanisms, the most important of which is the implicit definition of a hash value for each Oracle object. A hash value is a unique identifier used to identify an object. This identifier can be used to compare two objects for equality. Oracle uses hash values ​​to convert the equality operation into comparing two hash values. The following will introduce the process of achieving equality in Oracle in detail.

1. Hash function

The hash function is the algorithm used by Oracle to generate hash values. Hash functions use a fixed-length hash table to store hash values. A hash function calculates a fixed-length hash code based on an object's value. A hash code is an integer that uniquely identifies an object. When calculating a hash code, the hash function is calculated using the value of the object.

The advantage of the hash function is that it can quickly compare two objects for equality. By comparing the hashes of two objects, Oracle can quickly determine whether they are equal. If two objects have different hashes, then they must not be equal. If their hashes are the same, then they are probably equal. In this case, Oracle further compares the object's values ​​to determine whether they are equal.

2. Hash table

The hash table is a data structure used by Oracle to store the relationship between hash values ​​and objects. A hash table is a fixed-size array, and each array element contains a linked list and a lock. Each element in the hash table corresponds to a hash value, which is used to identify the uniqueness of the object. When an object needs to be found, Oracle calculates the hash value of the object and uses the hash value to locate the object in the hash table.

The advantage of a hash table is that it can quickly find objects. Through hash tables, Oracle can find objects in constant time. With linked lists, it can easily handle hash collisions and can handle large numbers of objects. With locks, it prevents multiple threads from accessing the same hash table element at the same time.

3. Application of equality

Equal applications in Oracle are very extensive. In SQL statements, the equality operator (=) is used to compare two values ​​for equality. In Oracle, in addition to the equality operator, there are other equality operations. The most commonly used ones include the IN and EXISTS operators. They are used to compare whether a value occurs in a set.

1. Equality operator

The equality operator is used to compare two values ​​for equality. The equality operator returns TRUE when two values ​​are equal in all respects; otherwise, it returns FALSE.

For example, if you need to query the employees with a salary of more than $5,000 in the employees table, you can use the following SQL statement:

SELECT * FROM employee WHERE salary >= 5000;

2. IN operator

The IN operator is used to compare whether a value is in a set. The IN operator returns TRUE when this value exists in the collection. Otherwise, it returns FALSE.

For example, if you need to query all employees working in department 10 or 30 in the employee table, you can use the following SQL statement:

SELECT * FROM employee WHERE department_id IN (10, 30);

3. EXISTS operator

The EXISTS operator is used to determine whether a query returns results. If the query returns results, the EXISTS operator returns TRUE. Otherwise, it returns FALSE.

For example, if you need to query whether employees working in department 10 exist, you can use the following SQL statement:

SELECT EXISTS (SELECT * FROM employee WHERE department_id = 10);

IV. Summary

This article introduces the meaning, implementation and application of equality in Oracle. Equality is achieved in Oracle using hash tables and hash functions. There are many applications of equality, including equality operator, IN operator and EXISTS operator. Equality is a very important concept in databases. Mastering the meaning, implementation and application of equality is very helpful for developing and maintaining efficient database applications.

The above is the detailed content of Discuss the meaning, implementation and application of equality in Oracle. For more information, please follow other related articles on the PHP Chinese website!