How to save data from dataset?
The modified data needs to be sent back to the database in order to save it. Therefore, to send the modified data to a database, the Update method of a TableAdapter or data adapter needs to be called. The Update method executes either of INSERT, UPDATE, or DELETE depending on the RowState in the table.
In Visual Studio a TableAdapterManager component is used for saving the proper order based on the foreign-key constraints of the database.
Although the procedure to save the data may change depending upon the applications, the following steps throw light on the generalized concept:
The code sends updates to the database should be written within a try/catch block.
The data row should be located to determine the problem area and the code should then be reattempted.
Database Interview: What is fillfactor? What is the use of it ? What happens when we ignore it? When you should use low fill factor?
When you create a clustered index, the data in the table is stored in
the data pages of the database according to the order of the values in
the indexed columns. When new rows of data are inserted into the table
or the values in the indexed columns are changed, Microsoft® SQL
Server™ 2000 may have to reorganize the storage of the data in the
table to make room for the new row and maintain the ordered storage of
the data. This also applies to nonclustered indexes. When data is
added or changed, SQL Server may have to reorganize the storage of the
data in the nonclustered index pages. When a new row is added to a
full index page, SQL Server moves approximately half the rows to a new
page to make room for the new row. This reorganization is known as a
page split. Page splitting can impair performance and fragment the
storage of the data in a table.
When creating an index, you can specify a fill factor to leave extra
Database Interview: What are statistics, under what circumstances they go out of date, how do you update them?
Statistics determine the selectivity of the indexes. If an indexed
column has unique values then the selectivity of that index is more,
as opposed to an index with non-unique values. Query optimizer uses
these indexes in determining whether to choose an index or not while
executing a query.
Some situations under which you should update statistics:
1) If there is significant change in the key values in the index
2) If a large amount of data in an indexed column has been added,
changed, or removed (that is, if the distribution of key values has
changed), or the table has been truncated using the TRUNCATE TABLE
statement and then repopulated
3) Database is upgraded from a previous version
SQL Optimization Tips
• Use views and stored procedures instead of heavy-duty queries.
This can reduce network traffic, because your client will send to
server only stored procedure or view name (perhaps with some
parameters) instead of large heavy-duty queries text. This can be used
to facilitate permission management also, because you can restrict
user access to table columns they should not see.
• Use table variables instead of temporary tables.
Table variables require less locking and logging resources than
temporary tables, so table variables should be used whenever possible.
The table variables are available in SQL Server 2000 only.
• Try to use UNION ALL statement instead of UNION, whenever possible.
The UNION ALL statement is much faster than UNION, because UNION ALL
statement does not look for duplicate rows, and UNION statement does
look for duplicate rows, whether or not they exist.
• Try to avoid using the DISTINCT clause, whenever possible.
Because using the DISTINCT clause will result in some performance
degradation, you should use this clause only when it is necessary.
• Try to avoid using SQL Server cursors, whenever possible.
SQL Server cursors can result in some performance degradation in
comparison with select statements. Try to use correlated sub-query or
derived tables, if you need to perform row-by-row operations.
• If you need to return the total table's row count, you can use
alternative way instead of SELECT COUNT(*) statement.
Because SELECT COUNT(*) statement make a full table scan to return the
total table's row count, it can take very many time for the large
table. There is another way to determine the total row count in a
table. You can use sysindexes system table, in this case. There is
ROWS column in the sysindexes table. This column contains the total
row count for each table in your database. So, you can use the
following select statement instead of SELECT COUNT(*): SELECT rows
FROM sysindexes WHERE id = OBJECT_ID('table_name') AND indid < 2 So,
you can improve the speed of such queries in several times.
• Include SET NOCOUNT ON statement into your stored procedures to stop
the message indicating the number of rows affected by a T-SQL statement.
This can reduce network traffic, because your client will not receive
the message indicating the number of rows affected by a T-SQL statement.
Explain about Clustered and non clustered index? How to choose between a Clustered Index and a Non-Clustered Index?
There are clustered and nonclustered indexes. A clustered index is a
special type of index that reorders the way records in the table are
physically stored. Therefore table can have only one clustered index.
The leaf nodes of a clustered index contain the data pages.
A nonclustered index is a special type of index in which the logical
order of the index does not match the physical stored order of the
rows on disk. The leaf nodes of a nonclustered index does not consist
of the data pages. Instead, the leaf nodes contain index rows.
Consider using a clustered index for:
o Columns that contain a large number of distinct values.
o Queries that return a range of values using operators such as
BETWEEN, >, >=, <, and <=. o Columns that are accessed sequentially. o Queries that return large result sets. Non-clustered indexes have the same B-tree structure as clustered indexes, with two significant differences: o The data rows are not sorted and stored in order based on their non-clustered keys. o The leaf layer of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows. Each index row contains the non-clustered key value and one or more row locators that point to the data row (or rows if the index is not unique) having the key value. o Per table only 249 non clustered indexes. 30. Disadvantage of index? Every index increases the time in takes to perform INSERTS, UPDATES and DELETES, so the number of indexes should not be very much. 31. Given a scenario that I have a 10 Clustered Index in a Table to all their 10 Columns. What are the advantages and disadvantages? A: Only 1 clustered index is possible.
Interview Questions: Indexes in SQL, What is Index? It’s purpose?
What is Index? It's purpose?
Indexes in databases are similar to indexes in books. In a database,
an index allows the database program to find data in a table without
scanning
the entire table. An index in a database is a list of values
in a table with the storage locations of rows in the table that
contain each value. Indexes can be created on either a single column
or a combination of columns in a table and are implemented in the form
of B-trees. An index contains an entry with one or more columns (the
search key) from each row in a table. A B-tree is sorted on the search
key, and can be searched efficiently on any leading subset of the
search key. For example, an index on columns A, B, C can be searched
efficiently on A, on A, B, and A, B, C.
Asp.net Interview: Can you explain the difference between an ADO.NET Dataset and an ADO Recordset?
In ADO, the in-memory representation of data is the recordset. In ADO.NET, it is the dataset. There are important differences between them.
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A recordset looks like a single table. If a recordset is to contain data from multiple database tables, it must use a JOIN query, which assembles the data from the various database tables into a single result table. In contrast, a dataset is a collection of one or more tables. The tables within a dataset are called data tables; specifically, they are DataTable objects. If a dataset contains data from multiple database tables, it will typically contain multiple DataTable objects. That is, each DataTable object typically corresponds to a single database table or view. In this way, a dataset can mimic the structure of the underlying database. A dataset usually also contains relationships. A relationship within a dataset is analogous to a foreign-key relationship in a database —that is, it associates rows of the tables with each other. For example, if a dataset contains a table about investors and another table about each investor’s stock purchases, it could also contain a relationship connecting each row of the investor table with the corresponding rows of the purchase table. Because the dataset can hold multiple, separate tables and maintain information about relationships between them, it can hold much richer data structures than a recordset, including self-relating tables and tables with many-to-many relationships.
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In ADO you scan sequentially through the rows of the recordset using the ADO MoveNext method. In ADO.NET, rows are represented as collections, so you can loop through a table as you would through any collection, or access particular rows via ordinal or primary key index. DataRelation objects maintain information about master and detail records and provide a method that allows you to get records related to the one you are working with. For example, starting from the row of the Investor table for "Nate Sun," you can navigate to the set of rows of the Purchase table describing his purchases. A cursor is a database element that controls record navigation, the ability to update data, and the visibility of changes made to the database by other users. ADO.NET does not have an inherent cursor object, but instead includes data classes that provide the functionality of a traditional cursor. For example, the functionality of a forward-only, read-only cursor is available in the ADO.NET DataReader object. For more information about cursor functionality, see Data Access Technologies.
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Minimized Open Connections: In ADO.NET you open connections only long enough to perform a database operation, such as a Select or Update. You can read rows into a dataset and then work with them without staying connected to the data source. In ADO the recordset can provide disconnected access, but ADO is designed primarily for connected access. There is one significant difference between disconnected processing in ADO and ADO.NET. In ADO you communicate with the database by making calls to an OLE DB provider. In ADO.NET you communicate with the database through a data adapter (an OleDbDataAdapter, SqlDataAdapter, OdbcDataAdapter, or OracleDataAdapter object), which makes calls to an OLE DB provider or the APIs provided by the underlying data source. The important difference is that in ADO.NET the data adapter allows you to control how the changes to the dataset are transmitted to the database — by optimizing for performance, performing data validation checks, or adding any other extra processing. Data adapters, data connections, data commands, and data readers are the components that make up a .NET Framework data provider. Microsoft and third-party providers can make available other .NET Framework data providers that can be integrated into Visual Studio.
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Sharing Data Between Applications. Transmitting an ADO.NET dataset between applications is much easier than transmitting an ADO disconnected recordset. To transmit an ADO disconnected recordset from one component to another, you use COM marshalling. To transmit data in ADO.NET, you use a dataset, which can transmit an XML stream.
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Richer data types.COM marshalling provides a limited set of data types — those defined by the COM standard. Because the transmission of datasets in ADO.NET is based on an XML format, there is no restriction on data types. Thus, the components sharing the dataset can use whatever rich set of data types they would ordinarily use.
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Performance. Transmitting a large ADO recordset or a large ADO.NET dataset can consume network resources; as the amount of data grows, the stress placed on the network also rises. Both ADO and ADO.NET let you minimize which data is transmitted. But ADO.NET offers another performance advantage, in that ADO.NET does not require data-type conversions. ADO, which requires COM marshalling to transmit records sets among components, does require that ADO data types be converted to COM data types.
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Penetrating Firewalls.A firewall can interfere with two components trying to transmit disconnected ADO recordsets. Remember, firewalls are typically configured to allow HTML text to pass, but to prevent system-level requests (such as COM marshalling) from passing.
General Database Interview Questions
In this segment, the interviewer may inquire about your general understanding about database management. It may include questions regarding definition of DDL and DML; sorting a table; different types of table join; definition of cursor; usages of cursor; the way of finding out the tables created by the user; implementation of locking; definition of lookup table in a database; about Datafile size; relation between free list and buffer busy wait; how to addressing modes; major differences between replication and logshipping; difference between database refresh and database cloning; etc.
Specific Questions
In this segment, the interviewer may ask you questions on the basis of your specialization, expertise and experience. The questions may vary depending on the database type. The interview questions may include questions on Btrieve, Database Admin, Informix, MYSQL, Progress, SQL, Stored Procedures, Teradata, Clipper, Firebird, Microsoft Access, PostgreSQL, SAP DB, SQL Server, Sybase, and Unify. Generally, in this segment, the applicants face questions from one or two categories depending on the job profile he has applied for.
Other Questions
In many cases, this type of job is mostly done in group setting. So during the interview process, the interviewer may ask you few relevant but other types of questions that may not be directly associated with your talent, skills or expertise. This type of questions may generally involve no correct or incorrect answers, but will reflect your idea about a specific ground. For example, the interviewer may ask you to assume a hypothetical situation when you find that you are working in such a group setting in which the relation between co-workers is not up to the mark, then what your state of action would be or he may ask you questions about your multi-tasking abilities, etc.
