-**Unique key constraint**: Ensures that uniqueness of a column entry or a combination of column entries are satisfied. Some unique keys are supposed to store aggregated information about its row. If this is the case, it can be seen in the description of the column. In the \__column_definition\_ view those columns are marked in the \__col_constraints\_ column with (u).
A unique key, in this case an identifier consits of several information of its row
identifier_analytical_method: Identifier of an analytical laboratory method defined in the table determ.analytical_methods. A laboratory method in best case is defined by a Standard (ISO or other) and consists of sample preparation and sample analysis techniques with appropriate measures for quality assurance and a validation procedure.
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-**Foreign key constraint**: Ensures the case sensitive referencing of a column or a combination of column entries in the so called "child table" originally migrated from the "parent table", hence providing referential integrity. In the \__column_definition\_ view those columns are marked in the \__col__constraints\_ column with (f).
<details><summary>Example</summary>
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<summary>Example</summary>
A foreign key references one or in this example two columns of another table case sensitive.
SQL_Umsetzung: ALTER TABLE hidden.samples ADD FOREIGN KEY (identifier_sampling_site, sampling_site_compartment) REFERENCES hidden.sampling_sites (identifier_sampling_site, compartment) ON UPDATE RESTRICT ON DELETE RESTRICT;
