inspiron 5378

the index, the number of rows of tables; The selection of the internal and external tables can be determined by the formula: number of matching rows in the outer table * the number of times each lookup in the inner-level table, the product is minimized as the best scheme Any action on a column in a 5.where clause results in a column-wise calculation at the SQL runtime, so it has to do a table search without using the index above the column, and if the results are available at query compile time

inner-level table, the product is minimized as the best schemeAny action on a column in a 5.where clause results in a column-wise calculation at the SQL runtime, so it has to perform a table search without using the index above the column, and if the results are available at query compile time, it can be optimized by the SQL optimizer, using the index, and avoiding table searches ( Example: SELECT * from record where substring (card_no,1,4) = ' 5378

and varchar, binary, and varbinary are incompatible. Incompatible data types may make the optimizer unable to perform some optimizations that could otherwise have been performed. For example:SELECT name from employee WHERE Salary > 60000In this statement, such as the salary field is a money type, it is difficult for the optimizer to optimize it because 60000 is an integer number. We should convert an integer into a coin type when programming, rather than wait for a run-time conversion.3. Try to

the inner table, the product is minimized as the best schemeAny action on a column in a 5.where clause results in a column-wise calculation at the SQL runtime, so it has to do a table search without using the index above the column, and if the results are available at query compile time, it can be optimized by the SQL optimizer, using the index, and avoiding table searches.Cases: The following is the referenced content:SELECT * FROM record where substring (card_no,1,4) = '

unknown and therefore cannot be selected as an input for the index. The following statement will perform a full table scan:Select ID from t where [email protected]You can force the query to use the index instead:Select ID from T with (index name) where [email protected]7. You should try to avoid expression operations on the fields in the WHERE clause, which will cause the engine to discard the full table scan using the index. Such as:SELECT * from T1 WHERE f1/2=100should read:SELECT * from T1 W

expression operations on the fields in the WHERE clause, which will cause the engine to discard the full table scan using the index. Such as:SELECT * from T1 WHERE f1/2=100should read:SELECT * from T1 WHERE f1=100*2 SELECT * from RECORD WHERE SUBSTRING (card_no,1,4) = ' 5378 'should read:SELECT * from RECORD WHERE card_no like ' 5,378% ' SELECT Member_number, first_name, last_name from membersWHERE DATEDIFF (Yy,datofbirth,getdate ()) > 21should read:

Basic commandsUseradd XXX Create a userUname Viewing system architecture informationUname-a Show DetailsUname-r displaying kernel informationDate Displays the current network timeCat/etc/redhat-release View Storage Release versionhostname xx Change host name XX (temporary effective)Ifconfig View IP AddressCat/proc/cpuinfo Viewing CPU informationCat/proc/meminfo Viewing memory informationcommand basic usage;LS: DisplayShort option:-l,-a,-c,-dLong option:--color,--author,--sortComposite options:-l

WHERE f1/2=100should read:SELECT * from T1 WHERE f1=100*2 SELECT * FROM record WHERE SUBSTRING (card_no,1,4) = ' 5378 'should read:SELECT * FROM record WHERE card_no like ' 5,378% ' SELECT Member_number, first_name, last_name fromWHERE DATEDIFF (Yy,datofbirth,getdate ()) > 21should read:SELECT Member_number, first_name, last_name fromWHERE dateOfBirth That is, any action on the column will result in a table scan, which includes database functions, ev

Characteristics:Length is changeable (bottom level is actually constant new array)You can manipulate multiple data types directlyWill eventually become a string through the ToString methodYou can modify a stringThe thread is synchronized The difference from the array: StringBuffer: Length is variable and can store different types of dataArray: The length is immutable, only one data type can be stored StringBuffer function: Storage (Create) StringBuffer append (): Adds the specified data as

types. For example, float and int, char and varchar, binary, and varbinary are incompatible. Incompatibility of data types may make the optimizer unable to perform some optimizations that could have been done. For example: SELECT name from employee WHERE Salary > 60000 In this statement, if the salary field is a money type, it is difficult for the optimizer to optimize it because 60000 is an integer number. Instead of waiting for run-time conversion, we should convert an integral type into a co

each lookup in the inner table, and the product minimum is the best scheme. Any operation result of a column in a 5.where clause is computed by column in the SQL runtime, so it has to do a table search instead of using the index above the column, and if the results are available at query compilation, you can optimize the SQL optimizer by using indexes to avoid table searches. Cases: The following are the referenced contents:SELECT * FROM record where substring (card_no,1,4) = '

SUBSTRING (card_no,1,4) = ' 5378 'should read:SELECT * FROM record WHERE card_no like ' 5,378% ' SELECT Member_number, first_name, last_name fromWHERE DATEDIFF (Yy,datofbirth,getdate ()) > 21should read:SELECT Member_number, first_name, last_name fromWHERE dateOfBirth That is, any action on the column will result in a table scan, which includes database functions, evaluation expressions, and so on, to move the action to the right of the equal sign wh

optimizer to use an index if necessary, such as using parameters in the WHERE clause, can also cause a full table scan. Because SQL resolves local variables only at run time, the optimizer cannot defer the selection of access plans to run time; it must be selected at compile time. However, if an access plan is established at compile time, the value of the variable is still unknown and therefore cannot be selected as an input for the index. The following statement will perform a full table scan:

expression operations on the fields in the WHERE clause, which will cause the engine to discard the full table scan using the index. Such as:SELECT * from T1 WHERE f1/2=100should read:SELECT * from T1 WHERE f1=100*2SELECT * from RECORD WHERE SUBSTRING (card_no,1,4) = ' 5378 'should read:SELECT * from RECORD WHERE card_no like ' 5,378% 'SELECT Member_number, first_name, last_name from membersWHERE DATEDIFF (Yy,datofbirth,getdate ()) > 21should read:SE

* from T1 WHERE f1/2=100should read:SELECT * from T1 WHERE f1=100*2SELECT * from RECORD WHERE SUBSTRING (card_no,1,4) = ' 5378 'should read:SELECT * from RECORD WHERE card_no like ' 5,378% 'SELECT Member_number, first_name, last_name from membersWHERE DATEDIFF (Yy,datofbirth,getdate ()) > 21should read:SELECT Member_number, first_name, last_name from membersWHERE dateOfBirth That is, any action on a column causes a table scan, which includes database