Database validation rules are difficult and a lot of the information about them on the internet assumes you are already an expert. Here are six to start with - along with validation text to go with them. From here, you will be able to look in to more complex validation rules and perhaps even make up a few of your own!
This might be a useful video if you are doing the Database Development course on the BTEC Level 2 or even L3 Diploma or Extended Certificate. However, be aware that these are just a starting point - use the logical operators you have previously encountered to work out many more validation rules. In other words if you want a certain rule then there is probably a way to express it in SQL. Don't forget that Microsoft Access users will also want to see a validation text - because if they are not told what they are doing wrong then how on earth are they meant to correct it?