The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint.
Indeed, What are SQL constraints?
SQL constraints are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.
Then, What are SQL indexes? An index contains keys built from one or more columns in the table or view. These keys are stored in a structure (B-tree) that enables SQL Server to find the row or rows associated with the key values quickly and efficiently. SQL Server documentation uses the term B-tree generally in reference to indexes.
What is NULL value? A null value in a relational database is used when the value in a column is unknown or missing. A null is neither an empty string (for character or datetime data types) nor a zero value (for numeric data types).
In the same way What is not null in SQL? The NOT NULL constraint enforces a column to not accept NULL values, which means that you cannot insert or update a record without adding a value to this field.
What are 5 types of constraints?
Types of Constraints in DBMS-
- Domain constraint.
- Tuple Uniqueness constraint.
- Key constraint.
- Entity Integrity constraint.
- Referential Integrity constraint.
What is cursor in SQL?
A cursor in SQL is a temporary work area created in system memory when a SQL statement is executed. A SQL cursor is a set of rows together with a pointer that identifies a current row. It is a database object to retrieve data from a result set one row at a time.
What is fragmentation in SQL?
SQL Server index fragmentation is a common source of database performance degradation. Fragmentation occurs when there is a lot of empty space on a data page (internal fragmentation) or when the logical order of pages in the index doesn’t match the physical order of pages in the data file (external fragmentation).
What is SQL optimization?
SQL Query optimization is defined as the iterative process of enhancing the performance of a query in terms of execution time, the number of disk accesses, and many more cost measuring criteria. Data is an integral part of any application.
What is view DBMS?
In SQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.
What is sequel language?
SQL (structured query language) is a language for specifying the organization of databases (collections of records). Databases organized with SQL are called relational, because SQL provides the ability to query a database for information that falls in a given relation.
What is SQL in DBMS?
SQL stands for Structured Query Language. It is a standard programming language for accessing a relational database. It has been designed for managing data in Relational Database Management Systems (RDBMS) like Oracle, MySQL, MS SQL Server, IBM DB2. SQL is one of the first commercial languages used for Edgar F.
What is left join SQL?
The LEFT JOIN command returns all rows from the left table, and the matching rows from the right table. The result is NULL from the right side, if there is no match.
How DELETE a column in SQL?
Right-click the column you want to delete and choose Delete Column from the shortcut menu. If the column participates in a relationship (FOREIGN KEY or PRIMARY KEY), a message prompts you to confirm the deletion of the selected columns and their relationships. Choose Yes.
What is PRIMARY KEY SQL?
The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).
What is key DBMS?
A key in DBMS is an attribute or a set of attributes that help to uniquely identify a tuple (or row) in a relation (or table). Keys are also used to establish relationships between the different tables and columns of a relational database. Individual values in a key are called key values.
What is join in DBMS?
In DBMS, a join statement is mainly used to combine two tables based on a specified common field between them. If we talk in terms of Relational algebra, it is the cartesian product of two tables followed by the selection operation.
What is tuple in DBMS?
(1) In a relational database, a tuple is one record (one row). See record and relational database. (2) A set of values passed from one programming language to another application program or to a system program such as the operating system.
What are the types of JOINs in SQL?
Different Types of SQL JOINs
- (INNER) JOIN : Returns records that have matching values in both tables.
- LEFT (OUTER) JOIN : Returns all records from the left table, and the matched records from the right table.
- RIGHT (OUTER) JOIN : Returns all records from the right table, and the matched records from the left table.
What are SQL views?
In SQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.
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