Create a Referential Integrity constraint.

Using the class example database (SQLFUN):

• Create a Referential Integrity constraint between the valid_phone_numbers table and the L_employees table. (This validates the phone_number value whenever a new row is added to the L_employees table.)
• Add one more value to this table: 4707.
• Create a new table, valid_phone_numbers, containing the phone numbers for all the employees. (Get this list of valid values from the phone_number column of the L_employees table.)

Goal: Use the date functions. Construct and use a table of constants.

• Find the number of days between December 21, 2020 (winter solstice), and March 21, 2021 (spring equinox).

Put both dates in a table of constants, so they both have a datatype of date.

Starting points: