How to query data volume in oracle

Oracle is a powerful database management system that is widely used in enterprise data management and processing. In Oracle, querying data is one of the most basic and common operations. This article will introduce how to query data volume in Oracle.

Overview

When dealing with large amounts of data, it is very important to understand the amount of data in the database. The Oracle database provides a variety of ways to query data volume to meet user needs.

Method 1: SELECT COUNT(*) statement

Use the SELECT COUNT(*) statement to count the number of records in the table. The syntax is as follows:

SELECT COUNT(*) FROM table_name;

where table_name is the name of the table to be queried.

For example, the statement to query the number of records in the table named employees is as follows:

SELECT COUNT(*) FROM employees;

This statement will return the total records in the employees table number.

Method 2: Use ROWNUM

ROWNUM is a pseudo column built into Oracle, used to represent the number of rows. The amount of data can be queried by processing ROWNUM.

For example, the statement to query the first 10 records in the table named employees is as follows:

SELECT * FROM (SELECT ROWNUM rn, t.* FROM employees t) WHERE rn <p>Among them, use the inner SELECT statement to process the ROWNUM column and name it <code>rn</code>, and then use the <code>WHERE</code> clause in the outer SELECT statement to select records that are <code>rn</code> less than or equal to 10. In this way, you can query the first 10 records in the <code>employees</code> table. </p><p>Similarly, the data volume can be queried by processing ROWNUM. For example, the statement to query the number of the first 100 records in the table named <code>employees</code> is as follows: </p><pre class="brush:php;toolbar:false">SELECT COUNT(*) FROM (SELECT ROWNUM rn, t.* FROM employees t) WHERE rn <p>Method 3: Use system tables</p><p>In Oracle, there are some system tables Can be used to query the amount of data in the database. One of the commonly used ones is <code>ALL_TABLES</code>, which lists all tables that the current Oracle user has access to. </p><p>For example, the statement to query the number of records in the table named <code>employees</code> is as follows: </p><pre class="brush:php;toolbar:false">SELECT num_rows FROM all_tables WHERE table_name='EMPLOYEES';

Among them, the num_rows column represents the number of records in the table . This statement will return the total number of records in the employees table.

It should be noted that because Oracle counts the number of records by traversing the entire table row by row, when processing large amounts of data, SELECT COUNT(*) and query methods using ROWNUM may take several minutes or It took several hours to complete the query.

Conclusion

Querying data volume in Oracle is a basic and important operation. This article introduces several different methods to query the number of data in the database, including using the SELECT COUNT(*) statement, ROWNUM, and system tables. Users should choose a suitable method to query the data volume according to their own needs.

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