SQL - INDEX Constraint


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The INDEX is used to create and retrieve data from the database very quickly. An Index can be created by using a single or group of columns in a table. When the index is created, it is assigned a ROWID for each row before it sorts out the data.

Proper indexes are good for performance in large databases, but you need to be careful while creating an index. A Selection of fields depends on what you are using in your SQL queries.

Example

For example, the following SQL syntax creates a new table called CUSTOMERS and adds five columns in it.

CREATE TABLE CUSTOMERS(
   ID   INT              NOT NULL,
   NAME VARCHAR (20)     NOT NULL,
   AGE  INT              NOT NULL,
   ADDRESS  CHAR (25) ,
   SALARY   DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

Now, you can create an index on a single or multiple columns using the syntax given below.

CREATE INDEX index_name
   ON table_name ( column1, column2.....);

To create an INDEX on the AGE column, to optimize the search on customers for a specific age, you can use the follow SQL syntax which is given below −

CREATE INDEX idx_age
   ON CUSTOMERS ( AGE );

DROP an INDEX Constraint

To drop an INDEX constraint, use the following SQL syntax.

ALTER TABLE CUSTOMERS
   DROP INDEX idx_age;

sql-rdbms-concepts.htm

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