Beginning Database Design- P9:This book focuses on the relational database model from a beginning perspective. The title is, therefore, Beginning Database Design. A database is a repository for data. In other words, you can store lots of information in a database. A relational database is a special type of database using structures called tables. Tables are linked together using what are called relationships. You can build tables with relationships between those tables, not only to organize your data, but also to allow later retrieval of information from the database | Reading and Writing Data with SQL An expression is a mathematical term representing any part of a larger mathematical expression. Thus an expression is an expression in itself can contain other expressions and can be a subset part of other expressions. So in the expression 5 3 23 - 50 5 3 is an expression so is 5 3 23 so is 5 3 23 and even the number 50 is an expression in this context. In the following expression however the conjunction of the second and third expressions is evaluated first then the result is evaluated against the first expression using the OR logical operator. This is because the AND operator has higher precedence than the OR operator expression1 OR expression2 AND expression3 Higher precedence implies executed first. The precedence of evaluation of expressions in the next expression is changed by using the parentheses. Therefore use of parentheses as in has higher precedence than NOT AND and OR. expression1 OR expression2 AND expression3 Aside from logical operator precedence there is also the factor of arithmetical precedence. Basic arithmetic is something we all learned in grade school mathematics. This is to refresh your memory rather than to insult your intelligence by explaining the completely obvious. Addition and subtraction have the lowest level of precedence but they are equal to each other 5 4 - 3 6 It should be plain to see why addition and subtraction have equal precedence because no matter what order in which the numbers are added and subtracted the result will always be the same. Try it out yourself in your head and you will understand better. Asking you to do an exercise like this in your head is once again not intended as an intellectual insult however just try it and you will understand how simplicity can be used to explain so many things. Perhaps even the answers to life itself could be answered so easily by breaking all questions into their constituent little pieces. The ability to break things into small parts to solve small
