COUNT()

Counts non-NULL values

Result type

BIGINT

Syntax

COUNT ([ALL | DISTINCT] <expr> | *)

Table 1. `COUNT` Function Parameters
Parameter	Description
expr	Expression.It may contain a table column, a constant, a variable, an expression, a non-aggregate function or a UDF that returns a numeric data type.Aggregate functions are not allowed as expressions

COUNT returns the number of non-null values in a group.

ALL is the default: it counts all values in the set that are not NULL.
If DISTINCT is specified, duplicates are excluded from the counted set.
If COUNT (*) is specified instead of the expression expr, all rows will be counted.COUNT (*) —
- does not accept parameters
- cannot be used with the keyword DISTINCT
- does not take an expr argument, since its context is column-unspecific by definition
- counts each row separately and returns the number of rows in the specified table or group without omitting duplicate rows
- counts rows containing NULL
If the result set is empty or contains only NULL in the specified column(s), the returned count is zero.

SELECT
  dept_no,
  COUNT(*) AS cnt,
  COUNT(DISTINCT name) AS cnt_name
FROM employee
GROUP BY dept_no