1 .
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Define Primary Key?
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The
primary key is the columns used to uniquely identify each row of a table.
A table
can have only one primary key.
No
primary key value can appear in more than one row in the table.
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2.
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Define Unique Key?
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Unique key is a one or more column that must be unique for each
row of the table.
It is similar to primary key. Primary key column will not accept a null. Whereas the unique key column will accept a null values. |
3.
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Define Foreign Key?
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A foreign Key is a combination of columns with value is based on
the primary key values from another table. A foreign key constraint also
known as Referential Integrity Constraint.
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4.
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Define View?
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A View
is a database object that is a logical representation of a table.
It is
derived from a table but has no longer of its own and often may be used in
the same manner as a table. A view is a virtual table that has columns similar to a table.
A view
does not represent any physical data.
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5.
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Compare and contrast TRUNCATE and DELETE for a table?
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Both the truncate and delete command have the desired outcome of
getting rid of all the rows in a table. The difference between the two is
that the truncate command is a DDL operation and just moves the high water
mark and produces a now rollback. The delete command, on the other hand, is a
DML operation, which will produce a rollback and thus take longer to
complete.
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6.
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What is cursors?
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Cursor is a database object used by applications to manipulate
data in a set on a row-by-row basis, instead of the typical SQL commands that
operate on all the rows in the set at one time.
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7.
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What is the difference between a HAVING CLAUSE and a WHERE
CLAUSE?
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HAVING can be used only with the SELECT statement. HAVING is
typically used in a GROUP BY clause. When GROUP BY is not used, HAVING
behaves like a WHERE clause. Having Clause is basically used only with the
GROUP BY function in a query. WHERE Clause is applied to each row before they
are part of the GROUP BY function in a query.
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8.
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What is the difference between TRUNCATE and DELETE commands?
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TRUNCATE is a DDL command whereas DELETE is a DML command. Hence
DELETE operation can be rolled back, but TRUNCATE operation cannot be rolled
back. WHERE clause can be used with DELETE and not with TRUNCATE.
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9.
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Define candidate key, alternate key, composite key?
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A candidate key is one that can identify each row of a table
uniquely. Generally a candidate key becomes the primary key of the table.If the table has more than one candidate key, one of them will become the
primary key, and the rest are called alternate keys.
A key formed by combining at least two or more columns is called composite key. |
