SQL All-in-One

Phases of SQL System Development

In developing any system, you start at the beginning and go through to the end, and it’s no different with SQL. The following list shows you what to consider at each phase of the SQL development lifecycle:

• Definition Phase: Precisely define the problem to be solved, its magnitude, and who will work on it.

• Requirements Phase: Develop a detailed description of exactly what the development effort will produce. Gather all relevant information and put it into a requirements document (Statement of Requirements). Get client signoff.

• Evaluation Phase: Determine exactly how you will meet the requirements. What tools will you use? How will you deploy your development team? Determine whether the job is doable within time and budget constraints.

• Design Phase: Create a database model and then design a database and database application that satisfy the terms of the requirements document.

• Implementation Phase: Build the database and the database application. Include copious documentation within the code and in external documents.

• Final Documentation and Testing Phase: Give the database and application a tough workout. Hit the system with every conceivable input condition and a few inconceivable ones. Try to overload it. See where it breaks. When it breaks, send it back to the implementers or even back to the designers. Document everything.

• Maintenance Phase: Fix latent bugs as they arise. Provide updates and enhancements called for by the client.

SQL Criteria for Normal Forms

In SQL, normal forms are defining characteristics of relational databases. SQL forms get classified according to the types of modification anomalies they’re subject to. First, second, and third normal forms (1NF, 2NF, 3NF) serve as remedies to the three main sources of modification anomalies.

The normal forms are nested in the sense that a table that’s in 2NF is automatically also in 1NF. Similarly, a table in 3NF is automatically in 2NF, and so on. For most practical applications, putting a database in 3NF is sufficient to ensure a high degree of integrity. To be absolutely sure of its integrity, you must put the database into DK/NF.

The table here lays out the criteria for each form:

SQL Value Functions

You use SQL value expressions to combine two or more values. Several kinds of SQL value expressions exist, corresponding to the different data types; the following table lists string, numeric and datetime values, functions, and effects:

SQL Set Functions

Sometimes, the information that you want to extract from an SQL table doesn’t relate to individual rows but rather to sets of rows. SQL provides five set (or aggregate) functions to deal with such situations — COUNT, MAX, MIN, SUM, and AVG. Each function performs an action that draws data from a set of rows rather than from a single row:

• COUNT: Returns the number of rows in the specified table

• MAX: Returns the maximum value that occurs in the specified table

• MIN: Returns the minimum value that occurs in the specified table

• SUM: Adds up the values in a specified column

• AVG: Returns the average of all the values in the specified column

How to Filter SQL Tables with WHERE Clauses

The WHERE clause is a filter that passes rows that meet the search condition and rejects rows that don’t meet the condition. By including WHERE clauses in SELECT statements, you can restrict the rows that you place into the resulting table to those that satisfy specific conditions. The following table lists the predicates to use to filter for the information you want:

SQL Data Types

Depending on their histories, different SQL implementations support a variety of data types. The SQL specification recognizes six predefined general types, shown in this table.