Knowing these values, we can find 1 or more records we need from our database. In the case of the country table, if we know the name of the country, we can easily find a record related to that country in our table and that way access all other values from the same record. As you can see, this is a pretty elegant way to access the data we need. This approach is used when we type search parameters on the front-end form and then pass that value to the SQL query.
Data from two related tables can be combined even if no primary key or foreign key constraints are defined between the tables, but a foreign key relationship between two tables indicates that the two tables have been optimized to be combined in a query that uses the keys as its criteria.
Although the main purpose of a foreign key constraint is to control the data that can be stored in the foreign key table, it also controls changes to data in the primary key table. For example, if the row for a salesperson is deleted from the Sales. SalesPerson table, and the salesperson's ID is used for sales orders in the Sales. SalesOrderHeader table, the relational integrity between the two tables is broken; the deleted salesperson's sales orders are orphaned in the SalesOrderHeader table without a link to the data in the SalesPerson table.
A foreign key constraint prevents this situation. The constraint enforces referential integrity by guaranteeing that changes cannot be made to data in the primary key table if those changes invalidate the link to data in the foreign key table. If an attempt is made to delete the row in a primary key table or to change a primary key value, the action will fail when the deleted or changed primary key value corresponds to a value in the foreign key constraint of another table.
To successfully change or delete a row in a foreign key constraint, you must first either delete the foreign key data in the foreign key table or change the foreign key data in the foreign key table, which links the foreign key to different primary key data. By using cascading referential integrity constraints, you can define the actions that the Database Engine takes when a user tries to delete or update a key to which existing foreign keys point.
The following cascading actions can be defined. For this constraint to execute, the foreign key columns must be nullable. For this constraint to execute, all foreign key columns must have default definitions. If a column is nullable, and there is no explicit default value set, NULL becomes the implicit default value of the column. If there are any AFTER triggers defined on the affected tables, these triggers fire after all cascading actions are performed.
These triggers fire in opposite order of the cascading action. If there are multiple triggers on a single table, they fire in random order, unless there is a dedicated first or last trigger for the table. However, one chain always fires all its triggers before another chain starts firing.
Specifically: A table can have only one primary key, and with very few exceptions, every table should have one. A primary key is implicitly UNIQUE - you cannot have more than one row with the same primary key, since its purpose is to uniquely identify rows.
Indexes exist for two reasons: To enforce a uniquness constraint these can be created implicitly when you declare a column UNIQUE To improve performance. Improve this answer. Michael Borgwardt Michael Borgwardt k 75 75 gold badges silver badges bronze badges.
A primary key is a constraint , not an index. An index can be associated with a primary key, but it's not required. And unless using SQL Server, an index doesn't ensure uniqueness. OMG: technically, the primary key constraint just like a unique constraint will always be implemented through an index. And an index ensures uniquness if it's declared so.
Oracle makes no such distinction about indexes, but the index on a pk can be removed without affecting referencial integrity. So no - primary keys are not implemented though an index. And the Postgres manual says "PostgreSQL automatically creates a unique index when a unique constraint or a primary key is defined for a table.
The index Show 5 more comments. Differences A table can only have one primary key , but several indexes. When to use what Each table should have a primary key.
Which is best? I recommend to always ask yourself first what the primary key of a table is and to define it.
The column s that make up the unique index can be nullable. There can be only one Primary Key defined on the table where as you can have many unique indexes defined on the table if needed. Also, in the case of SQL Server, if you go with the default options then a Primary Key is created as a clustered index while the unique index constraint is created as a non-clustered index. This is just the default behavior though and can be changed at creation time, if needed. Community Bot 1 1 1 silver badge.
The index key is composed of the same column s as the primary key. This index is created implicitly, as part of the primary key creation. In some databases, a PK index is created as a unique key index, to avoid the duplication of primary key values. At the same time, every query using a search criteria based on the primary key columns which frequently happens can be solved very quickly by the index.
For example:. When tables have many records, an index allows them to solve queries faster; however, to use an index there must be a match between the query search criteria and the index key.
Before closing, I want to take advantage of the fact that perhaps some readers are interested in a data engineering career. I suggest the article New Learning Path: Data Engineering to learn about what a data engineer needs to know.
Back to articles list Articles Cookbook. What is a primary key and why is it so important in SQL? Find out in this article. Tags: sql learn sql primary key.
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