Extract Row Metadata In MySQL 5.7.32

Extract Row Metadata In MySQL 5.7.32

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 InVisioneerJosh 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".

sql mysql

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