What SQL function is used to display a date by the day of the week?

The correct choice for displaying a date by the day of the week is the TO_CHAR function. This function is commonly used in SQL databases to convert a date or number to a specific string format, allowing you to display dates in various formats, including extracting the day of the week.

When using TO_CHAR with a date, you can specify the format model to return the desired representation of the date. For instance, using the format string 'Day' will return the full name of the day (e.g., "Monday"), while 'DY' might return the abbreviated name (e.g., "Mon"). This versatility in formatting makes TO_CHAR a powerful tool for presenting dates in a user-friendly manner.

The other options, while useful in different contexts, do not serve the purpose of formatting or displaying dates specifically by the day of the week. IS NULL is used for checking null values, CASE is a conditional expression for evaluating conditions, and NULLIF compares two expressions and returns null if they are equal. None of these functions would be appropriate for the task of formatting a date into a readable day of the week format.