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The Language of SQL- P6

The Language of SQL- P6:Research has shown that, being pressed for time, most readers tend to skip the introduction of any book they happen to read and then proceed immediately to the first real chapter. With that fact firmly in mind, we will only cover relatively unimportant material in the introduction, such as an explanation of what you will and will not learn by reading this book. | Looking Ahead 11 In the 1970s advances were made as hierarchical and network databases were invented and utilized. These newer databases through an elaborate system of internal pointers made it easier to read through data. For example a program could read a record for a customer automatically be pointed to all orders for that customer and then be pointed to all details for each order. But it was basically still the case that data needed to be processed one record at a time. The main problem with data storage prior to relational databases was not how the data was stored but how it was accessed. The real breakthrough with relational databases came when the language of SQL was developed because it allowed for an entirely new method of accessing data. Unlike earlier data retrieval methods SQL permitted the user to access a large set of data at a time. With one single statement a SQL command could retrieve or update thousands of records from multiple tables. This eliminated a great deal of complexity. Computer programs no longer needed to read one record at a time in a special sequence while deciding what to do with each record. What used to require hundreds of lines of programming code could now be accomplished with just a few lines of logic. Looking Ahead This chapter has provided enough background information about relational databases so that you can move on to the main topic which involves retrieving data from databases. We have discussed a number of important characteristics of relational databases such as primary keys foreign keys and datatypes. We also have talked about the possible existence of NULL values in data. We will add to our discussion of NULL values in Chapter 8 and return to the general topics of database maintenance in Chapter 18 and database design in Chapter 19. Why is the all-important topic of database design held off until much later in the book In the real world databases are designed and created before any data retrieval is attempted. Why .