must appear in the GROUP BY clause or be used in an aggregate function

This error message typically occurs in SQL when you are using a SELECT statement with GROUP BY but haven't included all the non-aggregated columns in the GROUP BY clause or used them in an aggregate function.

To fix this error, you have two options:

1. Include all non-aggregated columns in the GROUP BY clause: Make sure that all columns from the SELECT statement are either included in the GROUP BY clause or used in an aggregate function.

2. Use aggregate functions for non-aggregated columns: If including all non-aggregated columns in the GROUP BY clause is not feasible, you can use aggregate functions like SUM, COUNT, MAX, MIN, etc., for those columns instead.

Here's an example to illustrate the solutions:

-- Option 1: Include all non-aggregated columns in GROUP BY clause
SELECT column1, column2, SUM(column3)
FROM table
GROUP BY column1, column2;

-- Option 2: Use aggregate function for non-aggregated column
SELECT column1, column2, MAX(column3)
FROM table
GROUP BY column1, column2;

Remember to replace "column1", "column2", "column3", and "table" with your actual column names and table name.