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Databases SQL Reporting by Example Day 3: Getting Good at Grouping Busiest Teachers

Posted by lukej
lukej
34,222 Points

The meaning of COUNT(1)?

I am puzzled as to what the meaning of COUNT(1) is? It counts the first column or a column named 1?

I just avoid it by using COUNT(*), or COUNT(period_id) in this video

2 Answers

Steven Parker
Steven Parker
231,248 Points
Steven Parker
Steven Parker
231,248 Points

The "COUNT" function counts every row for which the supplied value is not null. So there's no difference between providing a "*", a literal digit, or an ID field. The number "1" is just a common convention used used to mean "count all rows".

But if you supply a field name that can contain nulls, rows where that field is null will not be counted. For examples, using the playground from this video:

SELECT COUNT(1) FROM subjects;      /* 24 (all rows) */
SELECT COUNT(name) FROM subjects;   /* 24 (no nulls) */
SELECT COUNT(grade) FROM subjects;  /* 18 (because of nulls) */
Tommy Gebru
Tommy Gebru
30,164 Points
Tommy Gebru
Tommy Gebru
30,164 Points

I also keep assuming it to be either the first column of the table or the first parameter of the SELECT statement