Usage of @ in mysql

In MySQL, the main uses of the @ symbol are: 1. Declare and obtain user variables; 2. Obtain the value of system variables; 3. Declaration and transfer of stored procedure parameters; 4. Check query cache hits; 5 . Create a temporary table.

Usage of @ symbol in MySQL

In MySQL, the @ symbol has the following usages:

1. User variables

@ symbols can be used to declare and obtain user variables. The syntax is:

<code>SET @variable_name = value;</code>

For example:

<code>SET @total_sales = (SELECT SUM(sales) FROM orders);</code>

2. System variables

@ symbols can also be used to obtain the values ​​of system variables. The syntax is:

<code>SELECT @@system_variable_name;</code>

For example:

<code>SELECT @@version;</code>

3. Stored procedure parameters

In stored procedures, the @ symbol is used to declare and pass parameters. The syntax is:

<code>CREATE PROCEDURE procedure_name (
    IN @param_name1 data_type,
    IN @param_name2 data_type,
    ...
);</code>

For example:

<code>CREATE PROCEDURE GetCustomerOrders (
    IN @customer_id INT
);</code>

4. Query cache hit

@ symbol can be used to check whether the query hits the query cache. The syntax is:

<code>SELECT /*!@SQL_CACHE*/ * FROM table_name;</code>

If the query hits the cache, the value of @SQL_CACHE is 1, otherwise it is 0.

5. Create a temporary table

@ symbol can be used to create a temporary table. The syntax is:

<code>CREATE TEMPORARY TABLE table_name (
    column_name1 data_type,
    column_name2 data_type,
    ...
)
;</code>

For example:

<code>CREATE TEMPORARY TABLE OrderSummary (
    order_id INT,
    product_id INT,
    quantity INT
)
;</code>

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