MySQL：The used SELECT statements have a different number of columns

執行SQL報錯：The used SELECT statements have a different number of columns

以上翻譯：使用的SELECT語句具有不同數量的列

原因：我們在 SQL 語句中使用了 UNION 連接兩張表時，查詢字段數量不一致導致

# 效果展示：

我們需要將數據展示如上圖所示

# 錯誤案例：

SELECT
	a.quantity AS in_quantity,
	a.price AS in_price,
	(a.quantity * a.price) AS in_amount,
	0 AS out_quantity,
	0 AS out_price,
	0 AS out_amount 
FROM
	store_in_detail a 
WHERE
	a.sku_id = 1345 
UNION ALL
SELECT
	b.quantity AS out_quantity,
	b.price AS out_price,
	(b.quantity * b.price) AS out_amount 
FROM
	store_out_detail b 
WHERE
	b.sku_id = 1345

我們通過入庫表 連接 出庫表，得出商品 id = 1345 的出入庫情況

執行SQL報錯：The used SELECT statements have a different number of columns (使用的SELECT語句具有不同數量的列)

# 原因分析：

我們在查詢入庫單，查詢了四個字段：入庫數量，入庫單價，入庫金額，出庫數量（默認0），出庫單價（默認0），出庫金額（默認0）
而查詢出庫單，只查詢了兩個字段：出庫數量，出庫單價，出庫金額

兩次查詢的字段數量不一致，導致 SQL 異常

# 正確實例：

SELECT
	a.quantity AS in_quantity,
	a.price AS in_price,
	(a.quantity * a.price) AS in_amount,
	0 AS out_quantity,
	0 AS out_price,
	0 AS out_amount 
FROM
	store_in_detail a 
WHERE
	a.sku_id = 1345 
UNION ALL
SELECT
	0 AS in_quantity,
	0 AS in_price,
	0 AS in_amount,
	b.quantity AS out_quantity,
	b.price AS out_price,
	(b.quantity * b.price) AS out_amount  
FROM
	store_out_detail b 
WHERE
	b.sku_id = 1345

SQL 執行成功

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