Querying Data from Table using SQL SELECTSummary in this tutorial, you will learn how to use the basic SQL SELECT statement to query data from a table.When you work with databases, querying data from a table is one of the most common tasks that you have to deal with on a regular basis.To query data from one or more tables, you use the SQL SELECT statement.SQL SELECT statement syntax.The following illustrates the SQL SELECT statement syntax ORDERBYcolumn.ASCDESC The SQL SELECT statement is made up of several clauses SELECT specifies which columns in database tables to include in the result or result set.FROM specifies the tables that you want to query data from.You can also use the FROM clause to join multiple tables.WHERE filters unwanted records or rows in the result.GROUP BY groups rows by common column values.HAVING uses with the GROUP BY clause to filter unwanted groups.V4hTLja7jU8/maxresdefault.jpg' alt='Update Select Multiple Rows Sql Tutorial' title='Update Select Multiple Rows Sql Tutorial' />ORDER BY sorts the result by one or more columns.The SQL SELECT statement only requires the SELECT clause to function properly.The other clauses e.FROM, WHERE, HAVING, GROUP BY, ORDER BY are optional.SQL-Concatenate-Rows-into-String-1.png' alt='Update Select Multiple Rows Sql Tutorial' title='Update Select Multiple Rows Sql Tutorial' />However, you always need to combine several clauses together to construct practical queries.In this tutorial, we are going to focus on the SELECT and FROM clauses.Lets get started.Select all data from a table.SQL Tutorial SELECT Statement Extended Query Capabilities This subsection details the remaining features of SELECT statements.The basics are at SELECT Statement.Before you want to query the data, ask yourself a question which table do I get the data from FROM clause.For example, to get all data from the employees table, you use the following query In this query, you put the employees table after the keyword FROM, and asterisk character after the SELECT keyword.The query asks the database server to return all rows and all columns of the employees table.The asterisk character is the shorthand of all columns.You can specify all the columns of the employees table explicitly in the SELECT clause as follows The result set is the same as the first query because all the columns of the employees tables are specified in the SELECT clause.Select a subset of columns in a table.In practice, you rarely get the data from all the columns of a table.You may only get the data from a subset of columns.In order to do so, you need to specify the needed columns in the SELECT clause.For example, if you want to get only last.Name of employees, you can use the following query If multiple columns are listed, they have to be separated by a comma, for example SQL SELECT statement with other elements.The SQL SELECT clause accepts not only the columns of a table but also the following elements Strings or Numbers.Expression. SQL function calls.User defined function calls.The following example illustrates how to use the SQL SELECT statement with a simple expression This example shows you how to combine a SQL string function named concat, which combines two or more string into one, to displays employees full name SELECTCONCATLast.Name, ,First. NameASfullname.A SQL alias is used to format the output in this example.In this tutorial, you have learned how to use the basic SQL SELECT statement to query data from table.In addition, you have also learned how to use SELECT statement with expression and function call.SQL Server cursor tutorial SQL Shack.Introduction. Most people that work with Microsoft SQL Server will have at least heard talk of cursors and often, even if people know on a basic level what SQL Server cursors do, they are not always certain when to use them and how to write the code behind them.So this article takes a step back and provides an explanation as to what SQL Server cursors can be used for as well as a basic example that you can run for yourself to test.Transactional versus Procedural Programming.SQL Server is a relational database management system RDBMS, and T SQL is a transactional programming language.This means that it is designed to execute its work in all or nothing runs.The database engine is optimized to work in this manner and, in general, it is more than sufficient to execute simple all in one type transactions.Many other programming languages including C and Visual Basic are iterative or procedural programming languages whereby the general flow of things is to treat each instance of an object separately and when dealing with many objects one would tend to loop over the same code until the stack is diminished and processed.Cursors however, like WHILE loops, break away from the transactional nature of T SQL and allow for programmers to treat each result of a SELECT statement in a certain way by looping through them.In the IT Engineering world it is common place for people to learn languages like C, VB, java, C or any other the other iterative type languages before having to deal with SQL in any realadvanced way.It is for this reason, and sadly so, that SQL Server cursors are often very prolific in some applications.It is a common trap that developers fall into and for good reason.The logic behind cursors can be perfect and the idea of writing one can seem good but one runs into real problems when it comes to performance because SQL Server is no longer about to treat whole chunks of data at once and instead has to repeat reads and writes for each result which can be catastrophic for IO performanceTherefore, as a general rule of thumb, and for good performance do not use cursors.However, there are some situations in which cursors can be lifesavers.I can think of a couple right off the bat Concurrent queries Sometimes, in OLTP On.Line Transaction Processing systems, there are just too many users actively querying a specific table.This is OK for small transactions as they are lightning fast and do not require locks on large chunks of the underlying tables.But in order to be able to update the entire table SQL Server often has to create a huge lock that blocks all other activity on the same table.This is in order to protect data consistency.If it was not the case a concurrent use could come along and SELECT rows from the table that are half updated and half not updated.There are server level options to handle the type of read commitments, these are called transaction isolation levels and they are outside of the scope of this article.However, if one has a READ COMMITTED transaction isolation level which is the case by default, SQL Server cursors or while loops can be helpful to break full table updates into multiple smaller batches.The second case where cursors may be useful is to create a for each type logic in T SQL scripts. Best Computer Diagnostic And Repair Software . For example, if one would like to handle the deployment of individual tables to another database one could use a cursor and spexecute.Sql to run a chunk of T SQL for each table in a give list.In the example below we will loop through the contents of a table and select the description of each ID we find.Take this simple table as an example.CREATE TABLE ITEMS ITEMID uniqueidentifier NOT NULL, ITEMDESCRIPTION VARCHAR2.NOT NULL. INSERT INTO ITEMS.NEWID, This is a wonderful car.NEWID, This is a fast bike.NEWID, This is a expensive aeroplane.NEWID, This is a cheap bicycle.NEWID, This is a dream holidayCREATE TABLEITEMS ITEMID uniqueidentifier NOT NULL, ITEMDESCRIPTION VARCHAR2.NOT NULLINSERT INTOITEMSVALUESNEWID,This is a wonderful car,NEWID,This is a fast bike,NEWID,This is a expensive aeroplane,NEWID,This is a cheap bicycle,NEWID,This is a dream holidayHere are the results of a SELECT FROM ITEMS query Now say we want to cut the selection of each description into 5 separate transactions.Here is the basic T SQL cursor syntax to do that.DECLARE ITEMID uniqueidentifier Here we create a variable that will contain the ID of each row.DECLARE ITEMCURSOR CURSOR Here we prepare the cursor and give the select statement to iterate through.OPEN ITEMCURSOR This charges the results to memory.FETCH NEXT FROM ITEMCURSOR INTO ITEMID We fetch the first result.WHILE FETCHSTATUS 0 If the fetch went well then we go for it.SELECT ITEMDESCRIPTION Our select statement here you can do whatever work you wish.WHERE ITEMID ITEMID In regards to our latest fetched ID.FETCH NEXT FROM ITEMCURSOR INTO ITEMID Once the work is done we fetch the next result.We arrive here when FETCHSTATUS shows there are no more results to treat.CLOSE ITEMCURSOR.DEALLOCATE ITEMCURSOR CLOSE and DEALLOCATE remove the data from memory and clean up the process.DECLAREITEMID uniqueidentifier Here we createavariable that will contain the ID of eachrow.DECLAREITEMCURSOR CURSOR Here we prepare the cursor andgive the select statement toiterate through.FORSELECT ITEMIDFROMITEMSOPEN ITEMCURSOR Thischarges the results tomemory.FETCH NEXT FROM ITEMCURSOR INTOITEMID We fetch the first result.WHILEFETCHSTATUS0 Ifthe fetch went well thenwe go forit.BEGINSELECT ITEMDESCRIPTION Our select statementhere you can dowhatever work you wishFROMITEMSWHERE ITEMIDITEMID Inregards toour latest fetched IDFETCH NEXT FROM ITEMCURSOR INTOITEMID Once the work isdone we fetch the next result.END We arrive here whenFETCHSTATUS shows there are no more results totreat.CLOSE ITEMCURSOR DEALLOCATE ITEMCURSOR CLOSE and.DEALLOCATE remove the data from memory andclean up the process.Running the basic SQL Server cursor above will loop through each ID in the ITEMS table and SELECT its corresponding ITEMDESCRIPTION in 5 separate transactions.You should get the following results after executing the cursor This example may see pointless but consider the fact that you can write any T SQL you like in between.WHILE FETCHSTATUS 0.FETCH NEXT FROM Cursor.NameWHILEFETCHSTATUS0.BEGINandFETCH NEXT FROMCursor.NameYou can certainly imagine many possible usages.Conclusion. This article is not meant to be used to proliferate the use of SQL Server cursors even more throughout your applications.As a general rule of thumb one should always think twice, even three times whether the use of a cursor is acceptable for their current problem.T SQL. However, as mentioned above, there are certain exceptions to that rule when it becomes perfectly acceptable to sacrifice performance to avoid blocking the application or just because there is no other option this is very rarely the case in ITSo, if you have determined that you absolutely need to use a SQL Server cursor, go ahead and build on the example above.Useful resources.SET TRANSACTION ISOLATION LEVEL Transact SQLUsing WHILEBREAK or CONTINUEHaving worked on highly transactional production systems and advanced corporate business intelligence, Evan is now using this experience make a difference in the e.Health world. He is driven by the love of technology and a desire to solve complex problems creatively.View all posts by Evan Barke.Latest posts by Evan Barke see all.
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