![]() Let’s use the payment table in the sample database for the demonstration. For example, we can use the COUNT() with the GROUP BY clause to return the number of films in each film category. We often use the COUNT() function with the GROUP BY clause to return the number of items for each group. In this form, the COUNT(DISTINCT column) returns the number of unique non-null values in the column. SELECT COUNT( column)Ĭondition Code language: SQL (Structured Query Language) ( sql ) COUNT(DISTINCT column) However, it does not consider NULL values in the column. Similar to the COUNT(*) function, the COUNT(column) function returns the number of rows returned by a SELECT clause. Because multiple transactions see different states of data at the same time, there is no direct way for COUNT(*) function to count across the whole table, therefore PostgreSQL must scan all rows. This is related to the PostgreSQL MVCC implementation. If you use the COUNT(*) function on a big table, the query will be slow. ![]() When you apply the COUNT(*) function to the entire table, PostgreSQL has to scan the whole table sequentially. SELECT COUNT(*)Ĭondition Code language: SQL (Structured Query Language) ( sql ) ![]() The COUNT(*) function returns the number of rows returned by a SELECT statement, including NULL and duplicates. ![]() The following statement illustrates various ways of using the COUNT() function. The COUNT() function is an aggregate function that allows you to get the number of rows that match a specific condition of a query. Summary: in this tutorial, you will learn how to use the PostgreSQL COUNT() function to count the number of rows in a table. ![]()
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