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DB
Interview Questions
| Q: |
What
is SQL? |
| A: |
SQL stands for 'Structured Query Language'. |
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| Q: |
What
is SELECT statement? |
| A: |
The SELECT statement lets you select a set of values
from a table in a database. The values selected
from the database table would depend on
the various conditions that are specified
in the SQL query. |
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| Q: |
How can you compare a part of the name rather than
the entire name? |
| A: |
SELECT * FROM people WHERE empname LIKE '%ab%'
Would return a recordset with records
consisting empname the sequence 'ab' in empname . |
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| Q: |
What is the INSERT statement?
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| A: |
The INSERT statement lets you insert information into
a database. |
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| Q: |
How do you delete a record from a database?
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| A: |
Use the DELETE statement to remove records or any particular
column values from a database. |
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| Q: |
How
could I get distinct entries from a table? |
| A: |
The SELECT statement in conjunction with DISTINCT lets
you select a set of distinct values from
a table in a database. The values selected
from the database table would of course
depend on the various conditions that are
specified in the SQL query. Example
SELECT DISTINCT empname FROM emptable |
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| Q: |
How
to get the results of a Query
sorted in any order? |
| A: |
You can
sort the results and return the sorted results
to your program by using ORDER BY keyword
thus saving you the pain of carrying out
the sorting yourself. The ORDER BY keyword
is used for sorting.
SELECT empname, age, city FROM emptable
ORDER BY empname |
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| Q: |
How can I find the total number of records
in a table? |
| A: |
You could use the COUNT keyword , example
SELECT COUNT(*) FROM emp WHERE age>40 |
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| Q: |
What
is GROUP BY? |
| A: |
The GROUP BY keywords have been added to SQL because
aggregate functions (like SUM) return the
aggregate of all column values every time
they are called. Without the GROUP BY functionality,
finding the sum for each individual group
of column values was not possible. |
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| Q: |
What
is the difference among "dropping a table",
"truncating a table" and "deleting all
records" from a table. |
| A: |
Dropping
: (Table structure
+ Data are deleted), Invalidates
the dependent objects
,Drops the indexes
Truncating:
(Data alone deleted), Performs
an automatic commit, Faster
than delete
Delete :
(Data alone deleted), Doesn’t perform
automatic commit
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| Q: |
What
are the Large object types suported by
Oracle? |
| A: |
Blob
and Clob. |
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| Q: |
Difference
between a "where" clause and
a "having" clause. |
| A: |
Having
clause is used only with group functions
whereas Where is not used with. |
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| Q: |
What's
the difference between a primary key and
a unique key? |
| A: |
Both
primary key and unique enforce uniqueness
of the column on which they are defined.
But by default primary key creates a clustered
index on the column, where are unique
creates a nonclustered index by default.
Another major difference is that, primary
key doesn't allow NULLs, but unique key
allows one NULL only. |
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| Q: |
What
are cursors? Explain different types of
cursors. What are the disadvantages of
cursors? How can you avoid cursors? |
| A: |
Cursors
allow row-by-row prcessing of the resultsets.
Types
of cursors: Static, Dynamic, Forward-only,
Keyset-driven. See books online for more
information.
Disadvantages
of cursors: Each time you fetch a row
from the cursor, it results in a network
roundtrip, where as a normal SELECT query
makes only one rowundtrip, however large
the resultset is. Cursors are also costly
because they require more resources and
temporary storage (results in more IO
operations). Furthere, there are restrictions
on the SELECT statements that can be used
with some types of cursors.
Most
of the times, set based operations can
be used instead of cursors.
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| Q: |
What
are triggers? How to invoke a trigger
on demand? |
| A: |
Triggers
are special kind of stored procedures that
get executed automatically when an INSERT,
UPDATE or DELETE operation takes place on
a table.
Triggers
can't be invoked on demand. They get triggered
only when an associated action (INSERT,
UPDATE, DELETE) happens on the table on
which they are defined.
Triggers
are generally used to implement business
rules, auditing. Triggers can also be
used to extend the referential integrity
checks, but wherever possible, use constraints
for this purpose, instead of triggers,
as constraints are much faster.
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| Q: |
What
is a join and explain different types
of joins. |
| A: |
Joins are used in queries to explain
how different tables are related. Joins
also let you select data from a table
depending upon data from another table.
Types of joins: INNER JOINs, OUTER JOINs,
CROSS JOINs. OUTER JOINs are further classified
as LEFT OUTER JOINS, RIGHT OUTER JOINS
and FULL OUTER JOINS.
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| Q: |
What
is a self join? |
| A: |
Self
join is just like any other join, except
that two instances of the same table will
be joined in the query. |
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