基础SQL语句-UPDATE

    总有一种感觉，就是csdn这个发表博客的页面真的是烂的可以。大家可以点击下面文字的超链接，直接看英语文档。基础的东西如果不及时回顾，还是会零星的忘记一些。所以看了一下update的操作。

UPDATE [LOW_PRIORITY] [IGNORE] table_reference
    SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
    [WHERE where_condition]
    [ORDER BY ...]
    [LIMIT row_count]

If you access a column from the table to be updated in an expression, UPDATE uses the current value of the column. For example, the following statement sets col1 to one more than its current value:

UPDATE t1 SET col1 = col1 + 1;

    下面的语句执行的结果是col1和col2拥有相同的值。单表的update操作从左到右执行。如果你更新的值等于目前的值，mysql并不会执行更新操作。

UPDATE t1 SET col1 = col1 + 1, col2 = col1;

    在NOT NULL类型的列被更新为NULL时，严格个SQL模式会发生错误。否则，该列会被设置成合法的默认值。如果是数值类型会被设置成0，字符串类型设置成空字符，以及日期和时间也会被设置成“zero”值。See Section 11.6, “Data Type Default Values”.

    更新操作返回受影响的行数。

    我们可以使用limit row_count来限制更新的范围。更新会在更新了符合where条件的row_count条数据之后停止操作。

    如果一个更新操作有order by语句，则更新会按照指定的顺序进行更新。这个操作在恰当的场合会很有用，尤其是更新唯一自增索引的时候，比如下面的sql语句就会有错误。

UPDATE t SET id = id + 1;

    举个例子，如果一个表包含1和2，在2更新为3之前就会发生error。去避免这个问题，增加一个order by语句，保证大的id更新在小的id之前。

UPDATE t SET id = id + 1 ORDER BY id DESC;

You can also perform UPDATE operations covering multiple tables. However, you cannot use ORDER BY or LIMIT with a multiple-table UPDATE. Thetable_references clause lists the tables involved in the join. Its syntax is described in Section 13.2.9.2, “JOIN Syntax”. Here is an example:

UPDATE items,month SET items.price=month.price
WHERE items.id=month.id;

The preceding example shows an inner join that uses the comma operator, but multiple-table UPDATE statements can use any type of join permitted inSELECT statements, such as LEFT JOIN.

If you use a multiple-table UPDATE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement fails and rolls back. Instead, update a single table and rely on the ON UPDATE capabilities that InnoDB provides to cause the other tables to be modified accordingly. See Section 14.8.6, “InnoDB and FOREIGN KEY Constraints”.

You cannot update a table and select from the same table in a subquery.

Index hints (see Section 8.9.3, “Index Hints”) are accepted for UPDATE statements, but are ignored prior to MySQL 5.5.6.

An UPDATE on a partitioned table using a storage engine such as MyISAM that employs table-level locks locks all partitions of the table. This does not occur with tables using storage engines such as InnoDB that employ row-level locking. This issue is resolved in MySQL 5.6. See Section 19.5.4, “Partitioning and Table-Level Locking”, for more information.