It is perfectly fine because in this case, MySQL considers NULL values are distinct. If a table has a PRIMARY KEY or UNIQUE NOT NULL index that consists of a single column that has an integer type, you can use _rowid to refer to the indexed column in SELECT statements, as follows: Another important point is that the UNIQUE constraint does not apply to NULL values except for the BDB storage engine. 0 not allowed in parent table and could work as NULL in binding table fine. For example, MySQL can use indexes and ranges to search for NULL with IS NULL. packed. MySQL UNIQUE index examples The MySQL IS NULL condition is used to test for a NULL value in a SELECT, INSERT, UPDATE, or DELETE statement.
mysql> SELECT * FROM my_table WHERE phone IS NULL; mysql> SELECT * FROM my_table WHERE phone = ''; See Section 3.3.4.6, “Working with NULL Values”, for additional information and examples.

MySQL IFNULL() Function MySQL Functions. indicates how the key is packed; NUL if it is not. Implied Conversions. A UNIQUE index permits multiple NULL values for columns that can contain NULL. This creates a nice small index of the rows where the column is NOT NULL. 11135. Al. Unlike other database systems, MySQL considers NULL values as distinct values. YES if the column may contain NULL values and blank if it does not. In addition, having NOT NULL places an extra integrity constraint on the data in the column, ensuring all rows have a value for the column. This MySQL tutorial explains how to use the MySQL IS NULL condition with syntax and examples. null. Why not use NOT NULL DEFAULT ''?Mysql treats NULLs as different,even from each other, in an unique index – Mihai Nov 27 '16 at 21:39 agree, but because it INT - DEFAULT 0 will be better. 24700. What you need is a filtered index using IS NOT NULL. MySQL performs the optimization for the IS NULL operator in the same way that it does for the equal (=) operator.. For example, MySQL uses the index when it searches for NULL with the IS NULL operator as shown in the following query:. comment. In fact, index invalidation and full table scanning are often caused by too many tables returned in a query. Heikki Tuuri. You can add an index on a column that can have NULL values if you are using the MyISAM, InnoDB, or MEMORY storage engine.

The index prefix. Views. Navigate: Previous Message• Next Message. Came across this and had some fun getting IS NOT NULL filtered indexes to work. SELECT customerNumber, salesRepEmployeeNumber FROM customers WHERE salesRepEmployeeNumber IS NULL; represents the index method used such as BTREE, HASH, RTREE, or FULLTEXT. Written By. Posted.

Options: Reply• Quote. Return the specified value IF the expression is NULL, otherwise return the expression: SELECT IFNULL(NULL, "W3Schools.com"); Try it Yourself » Definition and Usage. Example. …
当然也可以调整参数，使得null被认为是不同的值。-----怎么理解呢？-----先看空值跟null的区别。mysql官方： “NULL columns require additional space in the row to record whether their values are NULL. It is not a bug even though it was reported as a bug. Therefore, you can have multiple NULL values in the UNIQUE index. – a_vlad Nov 27 '16 at 21:43 null値が多いデータから、null値以外の検索をする IndexはNULLを含まないのでNULL値以外の検索には効果がある 逆に、Indexを作成すべきでない列の特徴 index_type. Subject. Examples: SELECT * FROM tbl_name WHERE key_col IS NULL; SELECT * FROM tbl_name WHERE key_col <=> NULL; SELECT * FROM tbl_name WHERE key_col = const1 OR key_col = const2 OR key_col IS NULL; This is how MySQL was designed. I think it turns out to be an issue with ANSI_NULLS in SQL… For example, take a large table with a sparsely populated column. Otherwise, it shows the number of indexed characters in case the column is partially indexed. If the expression is NOT NULL, this function returns the expression. Unique indexes and NULL column values. MySQL Forums Forum List ... Is there a way for InnoDB to behave the same as BDB when it comes to NULLs in unique indexes?

mysql calculates that the time cost of using an index is higher than that of a full table scan, so mysql prefers a full table scan to an index. The IFNULL() function returns a specified value if the expression is NULL. When the column is defined within an index, there is an improved size reduction and simplification of index processing, as no additional NULL conditions are required.