How to query the number of data rows in Oracle database

In Oracle database, querying the number of data rows is a very common operation. Querying the number of data rows is very useful when we need to know the amount of data in a table. This article will introduce how to query the number of data rows in an Oracle database.

Method 1: Use the COUNT function

The COUNT function is one of the functions that counts the number of rows in the Oracle database and can be used to query the number of data rows in a certain table. The specific syntax is as follows:

SELECT COUNT(*) FROM table_name;

Among them,table_namerepresents the name of the table for which the number of rows needs to be queried. The query results will show the total number of rows in the table.

For example, assuming we need to query the number of data rows in a table namedemployees, we can use the following statement:

SELECT COUNT(*) FROM employees;

This statement will returnemployeesThe total number of rows in the table.

It should be noted that the parameters in the COUNT function can be any expression, not necessarily*, so we can also count the number of rows in a specific field, for example :

SELECT COUNT(employee_id) FROM employees;

This statement will return the number of non-empty records in theemployee_idfield in theemployeestable.

Method 2: Use ROWNUM pseudo-column

There is a special pseudo-column in Oracle database named ROWNUM, which is used to return the row number of each row in the query result in order. Using ROWNUM, we can query the number of data rows. The specific syntax is as follows:

SELECT count(*) FROM (SELECT ROWNUM FROM table_name WHERE ROWNUM <= n);

Among them,table_namerepresents the name of the table where the number of rows needs to be queried, andnrepresents the number of rows that need to be queried. The main idea here is to query the first n rows in the table through the ROWNUM function, and then count the number of rows through the COUNT function.

For example, assuming we need to query the first 10 rows of data in a table namedemployees, we can use the following statement:

SELECT COUNT(*) FROM (SELECT ROWNUM FROM employees WHERE ROWNUM <= 10);

This statement will returnemployeesThe number of rows in the first 10 rows of data in the table.

It should be noted that when using the ROWNUM pseudo column to query the number of data rows, you need to pay attention to the order in which ROWNUM is used. ROWNUM must be used in the WHERE clause and must be used with the less than or equal operator, otherwise the query results will be incorrect.

Method 3: Use Oracle view ALL_TABLES

Oracle database provides a system viewALL_TABLES, which contains all table information that users can query, including table Name, user, table type, etc. TheNUM_ROWSfield in the ALL_TABLES view is used to query the number of data rows in each table. The specific syntax is as follows:

SELECT NUM_ROWS FROM ALL_TABLES WHERE TABLE_NAME = 'table_name';

Among them,table_namerepresents the name of the table for which the number of rows needs to be queried. This statement will return the number of rows in the table.

It should be noted that the number of data rows in the ALL_TABLES view may not be real-time because they are obtained from the data dictionary.

To sum up, the above three methods can all query the number of rows in the Oracle database. The COUNT function is the most commonly used method, which can directly query the number of rows in the table; the ROWNUM pseudo column can query the number of the first n rows; the ALL_TABLES view is more convenient, but the number of data rows it returns may not be real-time. According to actual needs, we can choose different methods to query the number of data rows.

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