![]() The following query is using simple CASE expression, where it checks the value of the column and returns the resultset as per value. In PostgreSQL, a Prepared Statement is a handy way to achieve better performance on extensively used queries, in practice avoiding the parse analysis step while allowing the execution plan to depend on the specific parameter values supplied. Let's use the following Employee table to understand the CASE expression. The data types of all the result expressions must be convertible to a single output type, otherwise CASE expression will raise error. If the ELSE clause is not defined for CASE expression, then it will return NULL. If of all the conditions evaluates to be false, then it returns else_result that is in the ELSE clause. If the condition evaluates to false, the CASE expression keeps on evaluating the next condition until it finds the expression to be evaluated as true. If the condition evaluates to true, the CASE expression will return the corresponding result set for that condition and stop evaluating the next expression. The CASE expression evaluates a list of conditions in sequence. In this tutorial, you have learned how to use the PostgreSQL COUNT() function to return the number of rows in a table.In the above syntax, every condition is a boolean expression that evaluates to be either true or false. HAVING COUNT (customer_id) > 40 Code language: SQL (Structured Query Language) ( sql ) For example, the following statement finds customers who have made more than 40 payments: SELECT You can use the COUNT function in a HAVING clause to apply a specific condition to groups. ![]() Here is the partial output: PostgreSQL COUNT() with HAVING clause The following query illustrates the idea: SELECTĬustomer_id Code language: SQL (Structured Query Language) ( sql ) To get the number of payments by the customer, you use the GROUP BY clause to group the payments into groups based on customer id, and use the COUNT() function to count the payments for each group. Payment Code language: SQL (Structured Query Language) ( sql ) PostgreSQL COUNT() with GROUP BY clause To get the distinct amounts which customers paid, you use the COUNT(DISTINCT amount) function as shown in the following example: SELECT COUNT ( DISTINCT amount) ![]() Here is the output: 2) PostgreSQL COUNT(DISTINCT column) example ![]() Payment Code language: SQL (Structured Query Language) ( sql ) The following statement uses the COUNT(*) function to return the number of transactions in the payment table: SELECT COUNT(*) 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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