oracle query alias

Oracle is a powerful relational database management system that supports many SQL query statements, such as SELECT, UPDATE, INSERT and DELETE. In actual database applications, aliasing is a common technology. It can make the query results more intuitive and easy to understand, while also reducing the number of input words and improving query efficiency. In this article, we will focus on how to use query aliases in Oracle database.

1. Basic syntax of query

In Oracle, the basic syntax of query statement is as follows:

SELECT [DISTINCT] column_name(s)
FROM table_name
WHERE condition(s);

Among them, the SELECT keyword indicates the column name to be queried, or it can Use the asterisk (*) to query all columns in the table. The FROM keyword indicates the table name to be queried, and the WHERE keyword indicates the query conditions. For example:

SELECT * FROM employees
WHERE department = 'Marketing';

The above statement queries all data in the employees table and limits the data rows whose department column is equal to 'Marketing'.

2. Query the alias

When querying the alias, you can use the AS keyword or omit it directly. Aliases can be used for any valid SQL identifier in a column name, table name, or subquery. For example:

SELECT first_name AS 'First', last_name AS 'Last'
FROM employees
WHERE department = 'Marketing';

The above statement will query all data in the employees table, rename the first_name column to 'First', rename the last_name column to 'Last', and then filter the data according to the condition department is equal to 'Marketing' .

It should be noted that when using aliases, they must be enclosed in single quotes or double quotes. If you use the AS keyword, you can omit the parentheses, as shown in the example above. If you omit the AS keyword, you must use parentheses to specify the alias, as in the following example:

SELECT first_name 'First', last_name 'Last'
FROM employees
WHERE department = 'Marketing';

The above statement is similar to the previous example, except that the AS keyword is omitted, but single quotes are used in the alias. This will query all the data in the employees table and rename the first_name column to 'First' and the last_name column to 'Last', and then filter the data based on the condition that department is equal to 'Marketing'.

To use an alias, you can reference the alias in the SELECT clause. For example:

SELECT first_name, last_name, salary AS 'Annual Salary'
FROM employees
WHERE department = 'Marketing';

The above statement queries all data in the employees table, renames the salary column to 'Annual Salary', and then filters the data according to the condition department equals 'Marketing'.

Similarly, when querying multiple tables, aliases can also be used to specify table names. For example:

SELECT employees.first_name, departments.department_name
FROM employees, departments
WHERE employees.department_id = departments.department_id;

The above statement queries the employees table and departments table, connects according to the condition that employees.department_id is equal to departments.department_id, and then queries according to the specified column name.

3. Summary

Query alias is a very commonly used technology in Oracle. It can make the query results more intuitive and easy to understand, while also reducing the number of input words and improving query efficiency. When using query aliases, you need to pay attention to the following points:

1. Use the AS keyword or omit it to specify the alias.
2. Must be enclosed in single or double quotes.
3. Aliases can be used for any valid SQL identifier in a column name, table name, or subquery.
4. Aliases can be referenced in the SELECT clause.
5. You can use aliases to specify table names when querying multiple tables.

I hope this article can help readers better understand the knowledge related to query aliases in Oracle.

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