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对大小写敏感,想忽略大小写可用CheckSum.
These functions are also valuable for comparing and indexing long string columns. Instead of creating an index on such column, you can add a column based on
the checksum or binary checksum of the value in the string column and index it instead. This type of index is typically called a hash index. SQL Server will
find matching records by comparing int values instead of comparing long strings, which might provide significant performance improvement.
对于包含长字符串的column，可以在该列值的checksum or binary checksum建索引 ，以代替直接在列值建索引，可提高performance。
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function Coalesce()
The Coalesce() function is often used to coalesce (unite) values that are split into several columns. The result of the function is the first non-null expression. This function uses the following syntax:
COALESCE(expression [,...n])
返回参数中第一个non-null值
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