Sybase 12.4.2 manual Using IQ Unique constraint on columns, Using Check conditions on columns, 278

Using table and column constraints

Using IQ UNIQUE constraint on columns

The IQ UNIQUE constraint specifies an estimate of the number of distinct values in a column. You can apply the IQ UNIQUE constraint to any column in a table. This constraint helps optimize loading of indexes.

For example, in the state column of the employee table, you would specify IQ UNIQUE(50) to indicate that there are only 50 possible values (assuming U.S. states only). Each of the possible values can occur many times.

Using CHECK conditions on columns

	You can use a CHECK condition to specify that the values in a column must
	satisfy some definite criterion.
	You can apply an unenforced CHECK condition to values in a single column,
	to specify the rules they should follow. These rules may be rules that data must
	satisfy in order to be reasonable, or they may be more rigid rules that reflect
	organization policies and procedures.
	CHECK conditions on individual column values are useful when only a
	restricted range of values are valid for that column. Here are some examples:
Example 1	You can specify a particular formatting requirement. If a table has a column for
	phone numbers you can specify that they all be entered in the same manner. For
	North American phone numbers, you could use a constraint such as the
	following:
	ALTER TABLE customer
	MODIFY phone
	CHECK ( phone LIKE ’(___) ___-____’ ) UNENFORCED
	Note The keyword UNENFORCED must appear after every CHECK condition.
Example 2
	You can specify that the entry should match one of a limited number of values.
	For example, to specify that a city column only contains one of a certain number
	of allowed cities (say, those cities where the organization has offices), you
	could use a constraint like the following:
	ALTER TABLE office
	MODIFY city
	CHECK ( city IN ( ’city_1’, ’city_2’, ’city_3’ ) )
	UNENFORCED

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