Why is SELECT * Considered Harmful in Database Programming?

*The Downsides of `SELECT `**

Why is using SELECT * generally discouraged in database programming? This seemingly simple query can introduce significant problems.

Reasons for Avoidance:

1. Performance Bottleneck: SELECT * retrieves every column from a table, even if the application only needs a few. This leads to excessive data transfer, impacting query speed and increasing server load.
2. Index Inefficiency: Including all columns can prevent the database from effectively using indexes. A carefully constructed index covering only the necessary columns dramatically improves performance, but SELECT * often bypasses this optimization.
3. Data Binding Challenges: When joining tables, SELECT * can lead to ambiguous column names (multiple columns with the same name), causing errors in the application's data handling. It also makes maintaining views more complex, as changes to the underlying tables can break the view.

*When `SELECT ` Might Be Acceptable:**

While generally avoided, there are limited exceptions:

1. Exploratory Data Analysis: For quick, informal data exploration, SELECT * offers a simple way to view all available data without needing to specify column names.
2. Row Counting or Existence Checks: In queries like SELECT COUNT(*) or EXISTS subqueries, * simply represents "a row," indicating the goal is to count rows or verify their existence, regardless of column values.

The above is the detailed content of Why is SELECT * Considered Harmful in Database Programming?. For more information, please follow other related articles on the PHP Chinese website!