Let us consider the below table which has the below values in it. And let us try to update the table values in the table based on some condition. Below is the statement that is for the same: —.
Let us see the table data. We can update the table rows based on the sub-query values as well. Below is example for the same: —. Here if we discuss the above query, here first the sub-query gets executed and returns the value. So to null a property, do this:. In the above answers, many ways and repetitions have been suggested for the same. I kept looking for an answer as mentioned is the question but couldn't find here.
It's a red herring.. Another possible reason for the empty string, rather than a true null is that the field is an index or is part of an index. This happened to me: using phpMyAdmin, I edited the structure of a field in one of my tables to allow NULLs by checking the " Null " checkbox then hitting the " Save " button.
That's when I realized that the field was part of the Primary key! I suspect the problem here is that quotes were entered as literals in your string value. You can set these columns to null using:. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How to update column with null value Ask Question.
Asked 11 years, 3 months ago. Active 1 year, 1 month ago. Viewed k times. Is there a special syntax to do this? Improve this question. Does the column allow a NULL value? No, not at all. It's really wierd — jim. Add a comment. This is because the order in which the rows are updated determines which rows are ignored.
Such statements produce a warning in the error log when using statement-based mode and are written to the binary log using the row-based format when using MIXED mode. Bug , Bug See Section For example, the following statement sets col1 to one more than its current value:. The second assignment in the following statement sets col2 to the current updated col1 value, not the original col1 value. The result is that col1 and col2 have the same value. This behavior differs from standard SQL.
For multiple-table updates, there is no guarantee that assignments are carried out in any particular order. If you set a column to the value it currently has, MySQL notices this and does not update it. For information about generated columns, see Section This can be useful in certain situations that might otherwise result in an error. Suppose that a table t contains a column id that has a unique index.
The following statement could fail with a duplicate-key error, depending on the order in which rows are updated:. For example, if the table contains 1 and 2 in the id column and 1 is updated to 2 before 2 is updated to 3, an error occurs. Its syntax is described in Section Here is an example:. In this case, the statement fails and rolls back. You cannot update a table and select directly from the same table in a subquery.
You can work around this by using a multi-table update in which one of the tables is derived from the table that you actually wish to update, and referring to the derived table using an alias. Suppose you wish to update a table named items which is defined using the statement shown here:.
As shown here, this statement does not work:. Instead, you can employ a multi-table update in which the subquery is moved into the list of tables to be updated, using an alias to reference it in the outermost WHERE clause, like this:.
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