Select COUNT (1) from table where: The function of this SQL statement

The function is to calculate how many rows match a condition.
1 does not represent the first field, but rather represents a fixed value, count (1) and COUNT (2) effect are the same

Count (*), the execution of the asterisk will be translated into the specific name of the field, the effect is the same, but more than a translation of the action, a fixed value is a little less efficient way.
Count (field value) will return a non-empty quantity that meets the criteria

1. Query all data, List 7 columns of data

2. Using the Count (1) query result, return 7

3. Using the Count (*) query result, return 7

4. Parentheses are the field values in the table, count (field values), return the number of data under the C_ID field in the table that is not empty

Returns the number of non-empty data that meets the criteria under the C_ID field