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Ans. Unique constraint ensures that a columns or combination or columns are always unique in a table and hence doesn't allow null or duplicates to be entered for the column or combination of columns.
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Ans. Application is unable to insert a record as it violates a unique constraint.
The exception states the constraint and Table can be located by the Entity mapping. So I will go to the DB and will first check to which all columns the unique constraint applies. And then I will go and check the code and logs to see how come the duplicate column values were attempted to be inserted when they were not supposed to be.
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Null value means that the value is yet to be assigned or need not be assigned whereas empty value means that the blank value has already been assigned once. Null is the default for the null able columns that hasn't be assigned any value.
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Which of the following is not true for Prepared Statements ?
Prepared Statements provides better performance
Prepared Statements prevent SQL Injection attacks
Prepared Statements provide ORM capabilities
Prepared Statement provides DB side caching
Q67. What are the benefits of using prepared statements ? Does that benefit extend only if similar prepared statements are used within a application or it extends even with multiple applications having similar prepared statement ?
Ans. Prepared Statements helps in reuse of database execution plan which can be cached at Database level and hence helps achieving faster execution and performance. As cached ps objects are creating at Database level and not application level, the use and benefit extends across multiple applications.
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Ans. Inner join is the intersection of two tables on the condition defined by the where clause i.e will get records from both tables matched by a column.
Outer join is the union of two tables i.e will get all records from both tables and will put null in the columns where related records are not present.
Left Outer join is the left union of two tables i.e all records from the table on the left and values from the right table for related records else null for the columns from right table.
Right Outer join is the right union of two tables i.e all records from the table on the right and values from the left table for related records else null for the columns from left table.
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Ans. JDBC is a standard Java Api for Database communication whereas Hibernate is an ORM framework that uses JDBC to connect with Database.
Hibernate is another layer of object table mapping over JDBC that provide a database independent, object oriented and an efficient way of database communication.
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Ans. Reliability and Continuity if some of the sites goes down.Easy Scaling up and Down as sites can be added or removed without impacting business continuity.
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Ans. A dirty read occurs when a transaction is allowed to read data from a row that has been modified by another running transaction but not yet committed.
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Ans. ACID stands for Atomicity, Consistency, Isolation, Durability is a set of properties of database transactions.
Atomicity means all or nothing. i.e parts of a transaction shouldn't commit if any one of them fails. Either the whole transaction should succeed or it should be complete rollback.
Consistency means that any transaction should lead database from one stabe state to another.
Isolation means that the execution of transaction results in a system state that would be obtained if transactions were executed serially.
Durability means that when a transaction is committed it forms the permanent state of database.
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Q86. There is a Table Student having Student Id, Name and Total Marks. Can you please write an SQL that will display the maximum marks obtained by a student ?
Ans. The external tables feature is a complement to existing SQL Loader functionality. It enables to access data in external sources as if it were in a table in the database. We have used it few times for replicating tables across different database systems.
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Which of the following is not true for Prepared Statements ?
