What is the keyword update?

UPDATE is a keyword used in SQL to modify data in a database table. It allows updating one or more rows of data in the table to change the value of the column. Common usages are: 1. Update a single column of a single row. ; 2. Update multiple columns; 3. Update all rows; 4. Update using subquery; 5. Update using JOIN.

UPDATE is a keyword in SQL used to modify data in a database table. It allows you to update one or more rows of data in a table to change the value of a column. Here are a few common uses of the UPDATE statement:

1. Update a single column of a single row:

UPDATE 表名   
SET 列名 = 新值   
WHERE 条件;

This will update the row in the table that meets the condition, and the specified column will be updated. The value of is changed to the new value.

2. Update multiple columns:

   UPDATE 表名
   SET 列1 = 新值1,
       列2 = 新值2,
       ...
   WHERE 条件;

This will simultaneously update the values ​​of multiple columns in the rows in the table that meet the conditions.

3. Update all rows:

   UPDATE 表名
   SET 列名 = 新值;

This will update all rows in the table, changing the value of the specified column to the new value.

4. Update using subquery:

   UPDATE 表名
   SET 列名 = (SELECT 子查询)
   WHERE 条件;

This will use the results of the subquery to update the rows in the table that meet the criteria.

5. Update using JOIN:

   UPDATE 表1
   INNER JOIN 表2 ON 条件
   SET 表1.列名 = 新值
   WHERE 条件;

This will join the two tables using the join condition and update the columns in Table 1 based on the condition.

Please note that before executing the UPDATE statement, please make sure you have backed up the database to prevent unexpected data changes. At the same time, depending on the specific situation, use the WHERE clause appropriately to limit the scope of updates to avoid meaningless or erroneous data modifications.

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