For the GROUP BY operator to work, must the field be present in the SELECT statement?

Yes, in most relational database management systems (SQL), the fields specified in the SELECT statement (except for aggregate functions) must be included in the GROUP BY clause.

Explanation:

The GROUP BY operator groups rows with the same values in the specified columns into a single summary row. The SELECT statement determines which columns will be displayed in the result. For the SELECT to correctly display values for each group, all non-aggregated columns in SELECT must be part of the grouping criteria specified in GROUP BY. Otherwise, the system won't know which specific value from the group to display.

Example:

Consider a table Orders with columns CustomerID, OrderDate, Amount.

CustomerID	OrderDate	Amount
1	2023-01-01	100
1	2023-01-15	200
2	2023-01-05	150

If we want to get the total sum of orders for each customer, we use GROUP BY CustomerID:

SELECT CustomerID, SUM(Amount)
FROM Orders
GROUP BY CustomerID;

Result:

CustomerID	SUM(Amount)
1	300
2	150

Here, CustomerID is present in both SELECT and GROUP BY. SUM(Amount) is an aggregate function and does not need to be included in GROUP BY.

Incorrect query:

If we try to include OrderDate in SELECT without including it in GROUP BY CustomerID:

SELECT CustomerID, OrderDate, SUM(Amount)
FROM Orders
GROUP BY CustomerID;

This query will cause an error because for CustomerID = 1, there are two different OrderDate values (2023-01-01 and 2023-01-15), and the system doesn't know which one to display for the aggregated group.

To make this query correct, either remove OrderDate from SELECT, or include it in GROUP BY:

SELECT CustomerID, OrderDate, SUM(Amount)
FROM Orders
GROUP BY CustomerID, OrderDate;

Exceptions/Syntax options (depends on the DBMS):

In some DBMSs (e.g., SQLite), there is a more lenient rule allowing non-aggregated columns in SELECT without including them in GROUP BY, if these columns are functionally dependent on the columns in GROUP BY (for example, if CustomerID is a PRIMARY KEY and determines all other columns in the table). However, this is not standard SQL and can lead to unpredictable results in other systems. Therefore, it's best to follow the general rule.