MySQL Keywords 


               


MySQL Keywords



PHP


In this article, we go over various MySQL keywords.

This is going to be ongoing list that will be constantly updated.

NOT NULL

The keywords NOT NULL can be declared in the definition of a MySQL column. Declaring a column NOT NULL means that the data entered into the rows of this column cannot be NULL, meaning there has to be a value entered into it. The data will not be entered into the MySQL table unless there is content being entered it. In other words, an actual value must be entered into the rows of this column. It cannot be a NULL value entered in.

Usually, you want a column to be NOT NULL. This is because you don't want empty strings in the table. However, in certain cases, you may accept empty strings from uses. In this case, you would simply not add the NOT NULL line to the column definition. If you use a service such as phpMyAdmin, without changing the settings, it will create all columns to be NOT NULL by default.

UNIQUE

The keyword UNIQUE can be declared in the definition of a MySQL column. Declaring a colum UNIQUE means that no values entered into the rows of this column can be duplicated. Therefore, each value must be unique.

You would declare column unique when no values in the row can be duplicated. Examples include things such as social security numbers, email addresses, possibly cell phone numbers. Since each individual is separate, they cannot have the same social security number, email address, etc. Therefore, in these situations, it is appropriate and rightful to make the column unique.

PRIMARY KEY

The keywords PRIMARY KEY can be declared in the definition of a MySQL column. No, one or multiple columns can be declared to be a PRIMARY KEY. Declaring a column a PRIMARY KEY means forces the column be NOT NULL and UNIQUE. This means that each row in a column must contain a value (it cannot be an empty string) and each row must have a unique value.

Usually, by convention, a MySQL table should have an ID column. Usually this column is declared to be a primary key that counts from 1 to how many rows there are. Usually, autoincrement goes in hand with this, so that each row is sequential. However, for certain tables, it may not be this way.





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