#Mysql database server good with numbers how to

Note that this version of the COUNT() function is rarely used because NULL rows should not be an issue in normalized databases, a condition that could only happen if the table didn't have a primary key. Selecting COUNT() from the table would return 2, even though there are 4 rows: SELECT COUNT(*) FROM code_values For example, say that we had a simple table called code_values: Invoking COUNT() in that way only returns rows which are not comprised of NULL values. Passing nothing to COUNT() executes the COUNT(expr) version of the function, but sans parameter. Counting only Non-null Rows with COUNT(expr) What that means is that running a query with COUNT(*) during a heavy workload could result in slightly inaccurate numbers. Consequently, SELECT COUNT(*) statements only count rows visible to the current transaction. If it did, concurrent transactions might “see” different numbers of rows at the same time. SELECT COUNT(*) FROM cities Ī statement like the one above that invokes the COUNT(*) function without a WHERE clause or additional columns, will perform very fast on MyISAM tables because the number of rows is stored in the table_rows column in the tables table of the information_schema database.įor transactional storage engines such as InnoDB, storing an exact row count is problematic because InnoDB does not keep an internal count of rows in a table.

That form of the COUNT() function basically returns the number of rows in a result set returned by a SELECT statement. To counts all of the rows in a table, whether they contain NULL values or not, use COUNT(*). In each case, COUNT() returns a BIGINT that contains either the number of matching rows, or zero, if none were found. The secret is in the function signatures, of which there are several forms: COUNT(*), COUNT(expression) and COUNT(DISTINCT expression). But there’s a little more to it than that, as the COUNT() function can be utilized to count all rows in a table or only those rows that match a particular condition. You probably already know that the COUNT() function returns the number of rows in a table.

#Mysql database server good with numbers how to

In part 2, we’ll learn how to obtain a row count from multiple tables, or even from all of the tables within a database. In today’s tip, we’ll use the native COUNT() function to retrieve the number of rows within one table or view within a MySQL database. Some database management products provide database statistics like table sizes, but it can also be done using straight SQL. There are several ways to get a row count in MySQL.