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Aqua Data Studio provides visual editors for creating database table Constraint objects. Constraints can be visually edited through the Create Table and Alter Table porperty windows, launched from a right click in the Schema Browser. Within the Alter and Create Table property windows, the Constraints Tab allows selection and editing of Table Constraints for Primary Key Constraints, Foreign Key Constraints, Unique Key Constraints and Check Constraints. After constraints have been set in the Constraints Tab, the Preview SQL tab displays the DDL used to generate them. When ER Models are generated from databases in the Schema Browser, constraints are visible within table entity objects. Primary Key Constraints A Primary key is a candidate key to uniquely identify each row in a table. A table is allowed one Primary Key constraint, and a column that is part of the Primary Key constraint cannot take null values. Because Primary Key constraints ensure unique data, they are most often defined on an identity column. Foreign key Constraints A foreign key is a referential constraint between two tables. The foreign key identifies a column or a set of columns in one (referencing) table that refers to a column or set of columns in another (referenced) table. The columns in the referencing table must be the primary key or other candidate key in the referenced table. The values in one row of the referencing columns must occur in a single row in the referenced table. Unique Constraints A unique constraint is a key which uniquely identifies all possible rows that exist in a table. Unique Constraints make sure that no identically copied values are entered in specific columns that do not participate in a primary key. Check Constraints A check constraint (also known as table check constraint) is a condition that defines valid data when adding or updating an entry in a table of a relational database. Within the Constraints Tab of Create or Alter Table, the Check Condition column is where the condition is set, determing what the table considers valid data for the selected column. |
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