将子查询存储在变量或其他东西中

-- 假设我们有一个订单表 orders 和一个客户表 customers
-- 我们想查询每个客户的订单总数

DECLARE @customer_id INT;
SET @customer_id = (SELECT customer_id FROM customers WHERE customer_name = 'John Doe');

SELECT * FROM orders WHERE customer_id = @customer_id;

-- 假设我们有一个订单表 orders 和一个客户表 customers
-- 我们想查询每个客户的订单总数

DECLARE @order_counts TABLE (
    customer_id INT,
    order_count INT
);

INSERT INTO @order_counts (customer_id, order_count)
SELECT customer_id, COUNT(*) AS order_count
FROM orders
GROUP BY customer_id;

SELECT * FROM @order_counts;

-- 假设我们有一个订单表 orders 和一个客户表 customers
-- 我们想查询每个客户的订单总数

CREATE TABLE #order_counts (
    customer_id INT,
    order_count INT
);

INSERT INTO #order_counts (customer_id, order_count)
SELECT customer_id, COUNT(*) AS order_count
FROM orders
GROUP BY customer_id;

SELECT * FROM #order_counts;

-- 使用完毕后删除临时表
DROP TABLE #order_counts;

DECLARE @customer_id INT;
SELECT @customer_id = customer_id FROM customers WHERE customer_name = 'John Doe';

IF @customer_id IS NOT NULL
BEGIN
    SELECT * FROM orders WHERE customer_id = @customer_id;
END
ELSE
BEGIN
    PRINT 'Customer not found';
END

DECLARE @order_count INT;
SET @order_count = (SELECT COUNT(*) FROM orders WHERE customer_id = 1);

SELECT @order_count AS order_count;