PostgreSQL COUNT Function
COUNT
The COUNT()
function returns the number of rows that matches a specified criterion.
If the specified criterion is a column name,
the COUNT() function returns the number of columns with that name.
Example
Return the number of customers from the customers table:
          SELECT COUNT(customer_id)
          FROM customers;
  
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Note: NULL values are not counted.
By specifying a WHERE clause, you can e.g. return the number of customers
that comes from London:
Example
Return the number of customers from London:
          SELECT COUNT(customer_id)
          FROM customers
          WHERE city = 'London';
  
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