This allows for multiple conditions to be checked in one location.ĬHECK constraints are similar to FOREIGN KEY constraints in that they control the values that are put in a column. For example, a multiple-column CHECK constraint could be used to confirm that any row with a country_region column value of USA also has a two-character value in the state column. You can also apply a single CHECK constraint to multiple columns by creating it at the table level. You can apply multiple CHECK constraints to a single column. The logical expression would be the following: salary >= 15000 AND salary <= 100000.
This prevents salaries from being entered beyond the regular salary range. For example, the range of values for a salary column can be limited by creating a CHECK constraint that allows for only data that ranges from $15,000 through $100,000. You can create a CHECK constraint with any logical (Boolean) expression that returns TRUE or FALSE based on the logical operators. CHECK ConstraintsĬHECK constraints enforce domain integrity by limiting the values that are accepted by one or more columns. Unless a clustered index is explicitly specified, a unique, nonclustered index is created by default to enforce the UNIQUE constraint. Therefore, if an attempt to insert a duplicate row is made, the Database Engine returns an error message that states the UNIQUE constraint has been violated and does not add the row to the table. The Database Engine automatically creates a UNIQUE index to enforce the uniqueness requirement of the UNIQUE constraint. If a UNIQUE constraint is added to a column that has duplicated values, the Database Engine returns an error and does not add the constraint. When a UNIQUE constraint is added to an existing column or columns in the table, by default, the Database Engine examines the existing data in the columns to make sure all values are unique. A UNIQUE constraint can be referenced by a FOREIGN KEY constraint. However, as with any value participating in a UNIQUE constraint, only one null value is allowed per column. Unlike PRIMARY KEY constraints, UNIQUE constraints allow for the value NULL. Although both a UNIQUE constraint and a PRIMARY KEY constraint enforce uniqueness, use a UNIQUE constraint instead of a PRIMARY KEY constraint when you want to enforce the uniqueness of a column, or combination of columns, that is not the primary key. For example, you can use UNIQUE constraints to make sure that no duplicate values are entered in specific columns that do not participate in a primary key. This topic contains the following sections.Ĭonstraints are rules that the SQL Server Database Engine enforces for you. As NIU's Graduate Catalog ( NIU, 2022b) emphasizes, research misconduct includes "falsification of data, improper assignment of authorship, claiming another person's work as one's own, unprofessional manipulation of experiments or of research procedures, misappropriation of research funds".UNIQUE constraints and CHECK constraints are two types of constraints that can be used to enforce data integrity in SQL Server tables. Good academic work is expected not only in classroom activities, but also in research and related activities. Students are responsible for plagiarism, intentional or not, if they copy material from books, magazines, or other sources without identifying and acknowledging those sources or if they paraphrase ideas from such sources without acknowledging them. Students are considered to have cheated if they copy the work of another during an examination or turn in a paper or an assignment written, in whole or in part, by someone else. Promoting honesty in academic work requires understanding the definition of academic dishonesty, its different types, and its causes and consequences.Īccording to NIU's undergraduate and graduate catalogs ( NIU, 2022b), The attempt of any student to present as his or her own work that which he or she has not produced is regarded by the faculty and administration as a serious offense. "Good academic work must be based on honesty" ( NIU, 2022a).
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