Postgres – Understanding COUNT in PostgreSQL

The SQL COUNT function or simply COUNT() is an aggregate function that returns the number of rows returned by a query. We can use this aggregate function in the SELECT statement to get a particular number of employees, the number of employees in each department, the number of employees who hold a specific job, etc. Basically, we use the count function to get the number of records required.
The result of the COUNT function depends on the argument that is passed to it.

By default, the COUNT function uses the ALL keywords whether you specify it or not. The ALL keywords mean that all items in the group are considered including the duplicate values. For example, if you have a group (1, 2, 3, 3, 4, 4) and apply the COUNT function, the result is 6.
If the DISTINCT keyword is specified explicitly, then only unique non-null values are considered. The COUNT function returns four if you apply it to the group (1,2,3,3,4,4).

A database is an organized collection of structured information, or data, typically stored electronically in a computer system. A database is usually controlled by a database management system (DBMS). Together, the data and the DBMS, along with the applications that are associated with them, are referred to as a database system, often shortened to just database.

Data within the most common types of databases in operation today is typically modeled in rows and columns in a series of tables to make processing and data querying efficient. The data can then be easily accessed, managed, modified, updated, controlled, and organized. Most databases use structured query language (SQL) for writing and querying data.
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