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After each Transact-SQL statement, the server sets the value of this variable to an integer value: 0 If the statement was successful Error number If the statement has failed This global variable is the foundation of all methods for error handling in the Microsoft SQL Server environment. It is essential to examine the value of this variable before any other Transact-SQL statement is completed, because the value of @@error will be reset. Even if the next statement is only a simple Select statement, the value of the @@error variable will be changed after it. In the following example, let s assume that an error will occur during the Update statement. @@error will contain the error code only until the next statement is executed; even the command for reading the @@error value will reset it. If it was completed successfully, SQL Server will set @@error to 0. The only way to preserve the @@error value is to immediately read it and store it in a local variable. Then it can be used for error handling.
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Update Equipment Set EqTypeId = 3 Where EqTypeId = 2 Select @intErrorCode = @@error
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If it is necessary to read more than one global variable immediately after a statement, all such variables should be included in a single Select statement:
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Declare @intEqId int, @intErrorCode int Insert into Equipment(Make, Model, EqTypeId) Values ('ACME', 'Turbo', 2) Select @intEqId = @@identity, @intErrorCode = @@Error
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The @@error variable will be set to an error number only in the case of errors, not in the case of warnings. Supplementary information that the server posts regarding errors or warnings (that is, severity, state, and error messages) are not available inside a stored procedure or a batch. Only the error number is accessible from a stored procedure or a batch. Further components of error messages can be read only from the client application. You will find more details about use of the @@error function in the Error Handling section in 7.
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After each Transact-SQL statement, the server sets the value of this variable to the total number of records affected by it. It can be used to verify the success of selected operations:
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select Make, Model, EqTypeid into OldEquipment from Equipment where EqTypeid = 2 if @@rowcount = 0 Print "No rows were copied!"
NOTE
Certain statements (like the If statement) will set @@rowcount to 0, and certain statements (like Declare) will not affect it.
Rowcount_big() is a function introduced in SQL Server 2000. It returns the number of affected records in the form of a bigint number.
When you try to update an individual record, SQL Server will not report an error if your Where clause specifies a criterion that does not qualify any records. SQL Server will not update anything, and you might, for example, think that the operation was successful. You can use @@rowcount to identify such cases.
Table Variables
SQL Server 2000 introduces the table data type. A statement declaring a variable for table initializes the variable as an empty table with a specified structure. As a table definition, such a statement includes definitions of columns with their data type, size, precision, optional primary key, unique and check constraints, and indexes. All elements have to be defined during the declaration. It is not possible to alter or add them later. The following batch declares a variable for table, inserts rows, and returns them to the user:
Declare @MyTableVar table (Id int primary key, Lookup varchar(15)) Insert @MyTableVar values (1, '1Q2000')
C h a p t e r 4 : B a s i c Tr a n s a c t - S Q L P r o g r a m m i n g C o n s t r u c t s
Insert @MyTableVar values (2, '2Q2000') Insert @MyTableVar values (3, '3Q2000') Select * from @MyTableVar Go
Because of their nature, table variables have certain limitations: table variables can only be part of the Select, Update, Delete, Insert, and Declare Cursor statements. table variables can be used as a part of the Select statement everywhere tables are acceptable, except as the destination in a Select...Into statement:
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