Using COUNT(), COUNT(column), And COUNT(expression) Variations To Extract Row Metadata In MySQL 5.7.32. Ben Nadel takes a quick look at the four variations of the COUNT() aggregate function in MySQL 5.7.32.
Yesterday, I was working with fellow InVisioneer, Josh Siok, to transform some MySQL data-tables into a common format. As we did this, we were using the
COUNT() aggregation function to gather metadata about the records that we were transforming.
COUNT() - and the other aggregation functions - are surprisingly flexible. As such, I thought it would be fun to take a quick look at the
COUNT() variations in MySQL 5.7.32.
In MySQL 5.x, there are four
COUNT() variations (depending on how you look at it):
COUNT( * ) - This counts all of the rows in the given result-set or
GROUP BY cohort. This variation does not care about the contents of the individual rows, only that they exist.
COUNT( column ) - This counts the number of non-NULL values that appear in the given column within the given result-set or
GROUP BY cohort.
COUNT( DISTINCT column ) - This counts the number of unique, non-NULL values that appear in the given column within the given result-set or
GROUP BY cohort.
COUNT( expression ) - This evaluates the given expression for each row within the given result-set or
GROUP BY cohort; and, counts the number of rows in which the expression evaluates to a non-
NULL result. This variation is super flexible and you can jam just about anything you want into the "expression".
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