Usage of * in sql

* in SQL is a wildcard character with the following usage: query all columns: SELECT * FROM table_name; alias for all columns: SELECT * AS all_columns FROM table_name; find specific values ​​in the WHERE clause: SELECT FROM table_name WHERE LIKE '%value%'; Used with aggregate functions: SELECT SUM(*) FROM table_name;

##Usage of * in SQL

The * (asterisk) in SQL is a wildcard character that has a wide range of uses in various queries.

Query all columns

The most common usage is to retrieve all columns in the table. For example:

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

This query will return the values ​​of all columns in the table.

Alias ​​for all columns

You can add an alias for * to easily reference all columns:

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

This query A virtual column named "all_columns" will be returned that contains all column values ​​in the table.

Use * in the WHERE clause

You can use * to find any column with a specific value in the WHERE clause. For example:

<code class="sql">SELECT * FROM table_name WHERE * LIKE '%value%';</code>

This query will return all column values ​​that contain the "value" string.

Using * with aggregate functions

You can use * with aggregate functions (such as SUM, COUNT) to calculate the sum or count of all columns. For example:

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

This query will return the sum of all numeric columns in the table.

Notes

• You should pay attention to the following when using *:
  It can cause query performance to degrade, Because all columns need to be retrieved.
  • It does not work with join queries across multiple tables.

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