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Table Handling
In RDBMS more than one table can be tackled at a time by using join operation. Join operation is a relational operation that causes 2 tables with a common domain to be joined into a single table or view. SQL states a join implicitly by referring the matching of common columns over which tables are connected in a WHERE clause. Two tables may be connected when each haves a column that shares a common domain with the other. The result of join operation is to be called a single table. Selected columns from all the tables are contained. Every row returned contains data from rows in the dissimilar input tables where values for the common columns match. A vital rule of table handling is that there should be one condition within the WHERE clause for every pair of tables being joined. Therefore if two tables are to be combined, one condition would be essential, but if three tables (X, Y, Z) are to be combined then two conditions would be necessary because there are two pairs of tables (X-Y and Y-Z) OR (X-Z and Y-Z), and so forth. There are various possible types of joins in relational database queries. Four types of join operations are define below:
Primary Index In primary index, there is a one-to-one relationship among the entries in the index table and the records in the major table. Primary index can be of two types:
Whenever the LIMIT clause will be used? LIMIT is used for the following kinds of problems: 1) Answering questions about first or last and largest or smallest 2) Splitting
What is foreign key? A relation schema r1 derived from an ER schema may contain among its attributes the primary key of another relation schema r2.this attribute is known as a
Update Anomaly : This anomaly is occurred due to data redundancy. Redundant information makes updates more complex since, for example, varying the name of the instructor of MCS-014
A Structure of a dependency diagram
ALTER TABLE Command : This command is used for variation of existing structure of the table in the given situation: When a new column is to be added to the table structure.
Designing the fields in the data base The following are the considerations one has to remain in mind while designing the fields in the data base. Coding, compression, en
How to Insert data in your table from another table? insert into e.g, To insert tuples from employee into N_emp created above, use following statement Insert into N_emp
Explain about primary key Primary Key: It is one of the candidate keys that are taken to be the recognizing key for the entire table. E.g. although there are two candidate ke
Mappings between Levels and Data Independence The 3 levels of abstraction in the database do not exist separately of each other. There must be some correspondence, or mapping
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