Books for you

Since Oracle8, the following constraint
types are supported:
NOT NULL
Youcan designate any column as NOT NULL. If any SQL operation leaves a
NULL value in a column with a NOT NULL constraint, Oracle returns an error
for the statement.
Unique
When you designate a column or set of columns as unique, users cannot add values
that already exist in another row in the table for those columns, or modify
existing values to match other values in the column.
The unique constraint is implemented by the creation of an index, which
requires a unique value. If you include more than one column as part of a unique
key, you will create a single index that will include all the columns in the unique
key. If an index already exists for this purpose, Oracle will automatically use that
index.
Constraints | 105
If a column is unique but allows NULL values, any number of rows can have a
NULL value, because the NULL indicates the absence of a value. To require a
truly unique value for a column in every row, the column should be both unique
and NOT NULL.


Pages:
222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246