That's pretty easy for people to grasp, once they have bought into the equals issue In the above example, the developer was expecting the inverse of the LIKE clause, that is, he expected the other 19, records to be returned, but if you look at the actual results, you see that what you get are only the records that have a value NOT NULL and that value is not like the search value.
There are a few different approaches you could take to fixing it. My first suggestion would be to disallow NULLs in the column in the first place. I'm like the ability to have NULL in many situations. To me there is a certain amount of truthiness to saying, "Hey, I don't know what the value is.
On the other hand, there are times when a column absolutely must have a value. Without a doubt, the created index is used by the database engine and it boosts the performance of the query because the logical reads number is lower than the previous one. The index seek operator details show us there is not any difference between the Actual Number of Rows and the Estimated Number of Rows. This situation shows that the query optimizer finds accurate information about how many NULL rows are stored in the MiddleName column.
Most likely, the following question will pop up in your mind. Basically, the statistics store the data distribution about the tables, and the query optimizer makes use of this information during the creation of a query plan. When we create an index this data distribution information is also stored for the NULL valued columns.
So that, optimizer estimates how much memory and other resources are required for a query. As we can see there is an number of NULL values are stored in the MiddleName column and this information is used by the query optimizer.
In some cases, the ISNULL function is used with the where condition but this usage method may lead to use indexes inefficiently. However, this query cannot use the created non-clustered index so it will read all index pages and then return the appropriate rows. To eliminate this situation, we can make a little code modification in the query so that the query optimizer can use the indexes more efficiently.
The following query returns some rows as like the previous one but it uses the indexes more efficiently. Determines whether a specific character string matches a specified pattern. A pattern can include regular characters and wildcard characters. During pattern matching, regular characters must exactly match the characters specified in the character string.
However, wildcard characters can be matched with arbitrary fragments of the character string. If any one of the arguments isn't of character string data type, the SQL Server Database Engine converts it to character string data type, if it's possible. When you do string comparisons by using LIKE, all characters in the pattern string are significant. Significant characters include any leading or trailing spaces.
If a comparison in a query is to return all rows with a string LIKE 'abc ' abc followed by a single space , a row in which the value of that column is abc abc without a space isn't returned. However, trailing blanks, in the expression to which the pattern is matched, are ignored. If a comparison in a query is to return all rows with the string LIKE 'abc' abc without a space , all rows that start with abc and have zero or more trailing blanks are returned. A string comparison using a pattern that contains char and varchar data may not pass a LIKE comparison because of how the data is stored for each data type.
The following example passes a local char variable to a stored procedure and then uses pattern matching to find all employees whose last names start with the specified set of characters. In the FindEmployee procedure, no rows are returned because the char variable EmpLName contains trailing blanks whenever the name contains fewer than 20 characters.
Because the LastName column is varchar , there are no trailing blanks. This procedure fails because the trailing blanks are significant. However, the following example succeeds because trailing blanks aren't added to a varchar variable.
0コメント