Because this constraint is disabled, no DML statements that modify the unique column are permitted against the sales table. You can use one of two strategies for modifying this table in the presence of a constraint:. Use DDL to add data to this table such as exchanging partitions. See the example in Chapter 15, " Maintaining the Data Warehouse". Before modifying this table, drop the constraint.
Then, make all necessary data modifications. Finally, re-create the disabled constraint. Re-creating the constraint is more efficient than re-creating an enabled constraint.
However, this approach does not guarantee that data added to the sales table while the constraint has been dropped is unique. A sample constraint might be:. The tables contain data that currently disobeys the constraint, but the data warehouse administrator wishes to create a constraint for future enforcement. Suppose that the data warehouse loaded new data into the fact tables every day, but refreshed the dimension tables only on the weekend.
The ETL process commonly verifies that certain constraints are true. For example, it can validate all of the foreign keys in the data coming into the fact table. This means that you can trust it to provide clean data, instead of implementing constraints in the data warehouse. You create a RELY constraint as follows:. This statement assumes that the primary key is in the RELY state.
RELY constraints, even though they are not used for data validation, can:. Enable more sophisticated query rewrites for materialized views.
See Chapter 18, " Query Rewrite" for further details. Enable other data warehousing tools to retrieve information regarding constraints directly from the Oracle data dictionary. Because the constraint is not being validated, no data processing is necessary to create it.
All constraints can be validated in parallel. When validating constraints on very large tables, parallelism is often necessary to meet performance goals. The degree of parallelism for a given constraint operation is determined by the default degree of parallelism of the underlying table. You can create and maintain constraints before you partition the data. Later chapters discuss the significance of partitioning for data warehousing. The unique key helps in uniquely identifying a record in the data table.
It can be considered somewhat similar to the Primary key as both of them guarantee the uniqueness of a record. But unlike the primary key, a unique key can accept NULL values and it can be used on more than one column of the data table.
We can see in the above image that when we tried to insert duplicate values, the entity constraints forced the server to throw errors. Referential Integrity Constraint ensures that there always exists a valid relationship between two tables. This makes sure that if a foreign key exists in a table relationship then it should always reference a corresponding value in the second table or it should be null.
We can create relationships between two tables in the following manner. There are a number of key constraints in SQL that ensure that an entity or record is uniquely or differently identified in the database. There can be more than one key in the table but it can have only one primary key. Integrity constraints in DBMS are a set of rules that are required to ensure consistency and integrity of data in the database.
A foreign key constraint is the main mechanism used to enforce referential integrity between tables in a relational database. A column defined as a foreign key is used to reference a column defined as a primary key in another table.
Check CHK constraints can be utilized to check the validity of data entered into particular table columns. Check constraints are used to provide back-end database edits, although edits are commonly found in the front-end application as well. General edits restrict values that can be entered into columns or objects, whether within the database itself or on a front-end application. The check constraint is a way of providing another protective layer for the data. Now study on-the-go. Find useful content for your engineering study here.
Questions, answers, tags - All in one app! Ask Question. Welcome back. Continue with Google Continue with email. Explain types of integrity constraints with example.
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