Primary Keys
The way you work with data tables in a relational database is a bit different from the way you
usually work on paper. A fundamental requirement in relational databases is that each data
row in a table must be uniquely identifiable. This makes sense because you usually save records
into a database so that you can retrieve them later; however, you can??™t always do that if each
table row doesn??™t have something that makes it unique. For example, suppose you add another
record to the department table shown previously in Figure 4-6, making it look like the table
shown in Figure 4-7.
Figure 4-7. Two departments with the same name
Look at this table, and tell me the description of the Seasonal department! Yep, we have
a problem??”we have two departments with the same name Seasonal (the name isn??™t unique).
If you queried the table using the name column, you would get two results. Sometimes getting
multiple results for a query is what you expect??”but other times you want the rows to be uniquely
identifiable depending on the value of a column, which is supposed to be unique.
This problem is addressed, in the world of relational database design, using the concept
of the primary key, which allows you to uniquely identify a specific row out of many rows.
Technically, the primary key is not a column itself. Instead, the PRIMARY KEY is a constraint that
when applied on a column guarantees that the column will have unique values across the table.
Pages:
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143