Constraint sql oracle example

NOT NULL column must have a value. UNIQUE Constraint column should have either unique values or null values. Primary Key Constraint. Foreign Key Constraint. It defines a field or set of fields where the combination must be unique in a table.

So, if you create a unique constraint on one column, all of the values in that column must be unique.

SQL constraints are used to specify rules for the data in a table. This ensures the accuracy and reliability of the data in the table. Constraints are used to limit the type of data that can go into a table. If there is any violation between the constraint and the data action, the action is aborted.

Check constraint controls the input of undesired data for each row in the table. This constraint enforce data integrity by defining logical expression for a column or columns. The true evaluation of expression validates the inserted or updated data.

I have explained about SQL Unique Constraint with examples also.

A unique constraint is an integrity constraint that ensures the data stored in a column, or a group of columns, is unique among the rows in a table. Typically, you apply the unique constraints to columns when you create the table using the inline constraint syntax as follows:. Enable a Check Constraint.

Use the exceptions_clause syntax to define exception handling. For example, if there is a UNIQUE constraint on coland colof a table, the combination of the values held by coland colwill be unique as long as these values are not. If one of coland colholds a NULL value, there can be another identical row in the table. A table can have multiple UNIQUE constraints. What are the different types of constraints in SQL?

What does the word constraint mean in SQL? Instead of permanently dropping a constraint from the database, you may want to temporarily disable the constraint and then enable it later. SQL Check Constraint is mostly used in so many real life scenarios in industry to restrict the value for the specific range. The primary key of a table is one of the candidate keys that you give some special characteristics.

You can have only one primary key, and a primary key column cannot contain NULLs. Drop Unique Constraint. This is the table that you wish to remove the unique constraint from. The name of the unique constraint to remove.

This example did not use the CONSTRAINT clause to explicitly assign the PRIMARY KEY constraint a name. Adding a primary key to a table.

Sometimes, you may want to add a primary key constraint to an existing table. For example , to drop the primary key constraint for a table in Oracle , you can use the following command. A CHECK CONSTRAINT on a table can refer to columns of the same table and not of any other table. No sub-query is allowed in the CHECK CONSTRAINT. This means that you cannot insert a new recor or update a record without adding a value to this field.

Notice the constraint keywor this indicates that we are getting ready to define a constraint. Recall that earlier in this chapter we discussed in-line and out of line constraints. This particular example is known as an in-line constraint because the constraint is being defined in the same line as the column being defined. 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 $10through $10000. This example uses the Create Table as Select to create a table from another table, using only some of the columns.

Here are some examples of Oracle alter table syntax to add foreign key constraints.