Setting Key to a Column For Quick Fetch Operation From Larze Records

To set a key to a column in SQL, you need to create an index on that column. An index is a data structure that allows the database to quickly look up records based on the values in the indexed column. By creating an index on a column, you can improve the performance of queries that filter, sort, or group by that column.

Here's an example of how to create an index on a column in SQL:

CREATE INDEX idx_my_column ON my_table (my_column);

creating index on a combination of columns

CREATE INDEX idx_my_column ON my_table (my_column1,my_column1);

This creates an index named "idx_my_column" on the "my_column" column of the "my_table" table. The index will allow the database to quickly fetch records based on the values in the "my_column" column.
To fetch a large number of records from a table with an indexed column, you can use a SELECT statement with a WHERE clause that filters on the indexed column. For example:

SELECT * FROM my_table WHERE my_column = 'some_value';

This will fetch all records from the "my_table" table where the value in the "my_column" column is equal to "some_value". The database will use the index to quickly find the matching records, which can significantly improve the performance of the query.
Note that creating an index on a column can have a cost in terms of storage and performance for write operations, as the database needs to maintain the index as the data changes. Therefore, it's important to carefully consider which columns to index based on the queries that will be executed against the table.