The meaning of * in sql

in SQL means all columns, it is used to simply select all columns in the table, the syntax is SELECT FROM table_name;. The advantages of using include simplicity, convenience and dynamic adaptation, but at the same time pay attention to performance, data security and readability. In addition, it can be used to join tables and subqueries.

The meaning of * in SQL

In SQL, * (asterisk) is a wildcard character that represents all List. It is often used to simply select all columns in a table without specifying column names.

How to use*

The syntax for using * is as follows:

<code class="sql">SELECT * FROM table_name;</code>

For example, the following query will select all columns from the "customers" table:

<code class="sql">SELECT * FROM customers;</code>

This will return rows containing all customer information, including customer ID, name, address, and phone number.

* Advantages

Using * has the following advantages:

• Conciseness:It makes the query more concise, Because you don't need to specify all column names.
• Convenience: It is especially convenient when there are a large number of columns in the table.
• Dynamic: It can adapt to changes in the table structure and does not require updating the query when adding or deleting columns.

* Precautions

The following should be noted when using *:

• Performance: Using * can cause performance issues when there is a large amount of data or columns in the table.
• Data Security: If the table contains sensitive data, you should avoid using * as it exposes all columns.
• Readability: In some cases, specifying column names can improve the readability of the query.

Other usages

In addition to selecting all columns, * can also be used to:

• Join tables: When joining tables using a JOIN statement, * can be used to select columns from both tables.
• Subquery: In a subquery, * can be used to select all columns in the rows returned by the parent query.

The above is the detailed content of The meaning of * in sql. For more information, please follow other related articles on the PHP Chinese website!