PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

236 Chapter 9 Creating Your Web Database
Table 9.4 MySQL Identifiers
Type Max Case Characters
Length Sensitive? Allowed
Database 64 same as OS Anything allowed in a directory name in your
OS except the /, \, and . characters
Table 64 same as OS Anything allowed in a filename in your OS
except the / and . characters
Column 64 no Anything
Index 64 no Anything
Alias 255 no Anything
These rules are extremely open.You can even have reserved words and special characters
of all kinds in identifiers.The only limitation is that if you use anything unusual like this,
you have to put it in backticks (located under the tilde key on the top left of most keyboards).
For example,
create database `create database`;
Of course, you should apply common sense to all this freedom. Just because you can
call a database `create database` doesn’t that mean that you should.The same principle
applies here as in any other kind of programming: Use meaningful identifiers.
Choosing Column Data Types
The three basic column types in MySQL are numeric, date and time, and string.Within
each of these categories are a large number of types.We summarize them here and go
into more detail about the strengths and weaknesses of each in Chapter 12.
Each of the three types comes in various storage sizes.When you are choosing a column
type, the principle is generally to choose the smallest type that your data will fit
into.
For many data types, when you are creating a column of that type, you can specify
the maximum display length.This is shown in the following tables of data types as M.If
it’s optional for that type, it is shown in square brackets.The maximum value you can
specify for M is 255.
Optional values throughout these descriptions are shown in square brackets.
Numeric Types
The numeric types are either integers or floating-point numbers. For the floating-point
numbers, you can specify the number of digits after the decimal place.This value is
shown in this book as D.The maximum value you can specify for D is 30 or M-2 (that is,
the maximum display length minus two—one character for a decimal point and one for
the integral part of the number), whichever is lower.