即使您现在希望选择每一列，也可能不希望在某人添加一个或多个新列之后选择每一列。如果您使用SELECT *来编写查询，那么在某些时候，您可能会冒这样的风险，即有人可能会添加一列文本，从而使您的查询运行得更慢，即使您实际上并不需要该列。

如果您添加了想要的新列，这难道不意味着需要更改的代码更少吗?

如果您确实想要使用新列，那么无论如何您都必须对代码进行大量其他更改。你只保存，new_column -只有几个字符的输入。

如果您真的想要每个列，我没有看到select(*)和命名列之间的性能差异。命名列的驱动程序可能只是为了明确您希望在代码中看到哪些列。

但是，通常情况下，您不希望每个列和select(*)会导致数据库服务器做不必要的工作，并且必须通过网络传递不必要的信息。它不太可能造成明显的问题，除非系统被大量使用或网络连接很慢。

有三个主要原因:

Inefficiency in moving data to the consumer. When you SELECT *, you're often retrieving more columns from the database than your application really needs to function. This causes more data to move from the database server to the client, slowing access and increasing load on your machines, as well as taking more time to travel across the network. This is especially true when someone adds new columns to underlying tables that didn't exist and weren't needed when the original consumers coded their data access. Indexing issues. Consider a scenario where you want to tune a query to a high level of performance. If you were to use *, and it returned more columns than you actually needed, the server would often have to perform more expensive methods to retrieve your data than it otherwise might. For example, you wouldn't be able to create an index which simply covered the columns in your SELECT list, and even if you did (including all columns [shudder]), the next guy who came around and added a column to the underlying table would cause the optimizer to ignore your optimized covering index, and you'd likely find that the performance of your query would drop substantially for no readily apparent reason. Binding Problems. When you SELECT *, it's possible to retrieve two columns of the same name from two different tables. This can often crash your data consumer. Imagine a query that joins two tables, both of which contain a column called "ID". How would a consumer know which was which? SELECT * can also confuse views (at least in some versions SQL Server) when underlying table structures change -- the view is not rebuilt, and the data which comes back can be nonsense. And the worst part of it is that you can take care to name your columns whatever you want, but the next guy who comes along might have no way of knowing that he has to worry about adding a column which will collide with your already-developed names.

但这对SELECT *来说也不全是坏事。我在以下用例中大量使用它:

Ad-hoc queries. When trying to debug something, especially off a narrow table I might not be familiar with, SELECT * is often my best friend. It helps me just see what's going on without having to do a boatload of research as to what the underlying column names are. This gets to be a bigger "plus" the longer the column names get. When * means "a row". In the following use cases, SELECT * is just fine, and rumors that it's a performance killer are just urban legends which may have had some validity many years ago, but don't now: SELECT COUNT(*) FROM table; in this case, * means "count the rows". If you were to use a column name instead of * , it would count the rows where that column's value was not null. COUNT(*), to me, really drives home the concept that you're counting rows, and you avoid strange edge-cases caused by NULLs being eliminated from your aggregates. Same goes with this type of query: SELECT a.ID FROM TableA a WHERE EXISTS ( SELECT * FROM TableB b WHERE b.ID = a.B_ID); in any database worth its salt, * just means "a row". It doesn't matter what you put in the subquery. Some people use b's ID in the SELECT list, or they'll use the number 1, but IMO those conventions are pretty much nonsensical. What you mean is "count the row", and that's what * signifies. Most query optimizers out there are smart enough to know this. (Though to be honest, I only know this to be true with SQL Server and Oracle.)

我不认为这真的可以有一个通用的规则。在许多情况下，我避免使用SELECT *，但我也使用过SELECT *非常有用的数据框架。

和所有事情一样，有收益也有成本。我认为收益与成本的等式的一部分是你对数据结构有多少控制。在SELECT *工作良好的情况下，数据结构受到严格控制(它是零售软件)，因此没有太大的风险，有人会将一个巨大的BLOB字段发送到表中。

一般来说，你必须匹配你的SELECT *…不同类型的数据结构。如果不指定结果到达的顺序，正确排列所有内容可能会很棘手(而且更模糊的字段更容易错过)。

通过这种方式，您可以出于各种原因向表中添加字段(甚至在表中间)，而不会破坏整个应用程序中的sql访问代码。

可以把它看作是减少应用程序和数据库之间的耦合。

总结一下“代码气味”方面: SELECT *在应用程序和模式之间创建一个动态依赖关系。限制它的使用是使依赖更加明确的一种方法，否则对数据库的更改更有可能使应用程序崩溃。