用圆括号将选定内容括起来。

SET @v1 := (SELECT COUNT(*) FROM user_rating);
SELECT @v1;

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此外，如果你想通过一个查询一次设置多个变量，你可以使用另一种语法来设置变量，如下所示：SELECT @varname:=value。
一个实际的例子：

SELECT @total_count:=COUNT(*), @total_price:=SUM(quantity*price) FROM items ...

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使用此

SELECT weight INTO @x FROM p_status where tcount='value' LIMIT 1;

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测试和工作正常.

Select count(*) from table_name into @var1; 
Select @var1;

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如果您想将查询结果保存为临时表，以便在后续查询中重用，请使用MySQL CREATE TEMPORARY TABLE。
请参阅：https://dev.mysql.com/doc/refman/8.0/en/create-temporary-table.html

-- begin: drop first just in case
DROP TEMPORARY TABLE IF EXISTS myDb.temp_variable; 
CREATE TEMPORARY TABLE myDb.temp_variable SELECT * FROM myDb.student ORDER BY id DESC LIMIT 25;

-- your scripts
SELECT * FROM myDb.temp_variable;

-- end: drop to prevent cached values accessed by other queries
DROP TEMPORARY TABLE IF EXISTS myDb.temp_variable;

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如果你想在SELECT... WHERE... IN (*here*)中使用变量。
使用GROUP_CONCAT和find_in_set。

-- initialise with a comma-separated string
SET @emails = '[email protected],[email protected],[email protected]';

-- search condition
SELECT * FROM customer WHERE find_in_set(email, @emails);

-- or, you can initialise the variable using query results
SELECT GROUP_CONCAT(phone) INTO @activeCustomerPhones FROM customer WHERE status = 'ACTIVE';

SELECT * FROM transaction WHERE find_in_set(phone, @activeCustomerPhones);

/* 
customer:
- phone
- email
- status 

transaction:
- phone
*/

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