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Database administrators are in a similar position as they might know the database schema but do not have deep insight into the access paths.The only place where the technical database knowledge meets the functional knowledge of the business domain is the development department.The database considers each column according to its position in the index definition to sort the index entries.The first column is the primary sort criterion and the second column determines the order only if two entries have the same value in the first column and so on.Note that the column order of a concatenated index has great impact on its usability so it must be chosen carefully.

I need to produce mailing labels from my SQL Server database so I am using the sign for concatenation the first, middle, and last names together. The limitation of this method is if any of the fields you are concatenating are NULL, the entire result is NULL.If not, the index tree cannot help find that place. We can take advantage of the fact that the first index column is always usable for searching.Again, it is like a telephone directory: you don't need to know the first name to search by last name. SUBSIDIARY_ID = 00002.) ---- ----------- ------ --------- --------- ------ ------- | id | table | type | key | key_len | rows | Extra | ---- ----------- ------ --------- --------- ------ ------- | 1 | employees | ref | PRIMARY | 5 | 123 | | ---- ----------- ------ --------- --------- ------ ------- -------------------------------------------------------------- |Id |Operation | Name | Rows | Cost | -------------------------------------------------------------- | 0 |SELECT STATEMENT | | 106 | 75 | | 1 | TABLE ACCESS BY INDEX ROWID| EMPLOYEES | 106 | 75 | |*2 | INDEX RANGE SCAN | EMPLOYEE_PK | 106 | 2 | -------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("SUBSIDIARY_ID"=20)In general, a database can use a concatenated index when searching with the leading (leftmost) columns.First, the code below is the old technique to perform concatenation using the sign: As you can see in the screen shot below the Mailing Name is NULL for any row that has NULL for any one of the name columns.The only rows that have Mailing Name filled in have a value for all the title, firstname, middlename, and lastname columns.

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