本文實例講述了mysql多個left join連接查詢用法。分享給大家供大家參考，具體如下：
mysql查詢時需要連接多個表時，比如查詢訂單的商品表，需要查詢商品的其他信息，其他信息不在訂單的商品表，需要連接其他庫的表，但是連接的條件基本都是商品ID就可以了，先給一個錯誤語句（查詢之間的嵌套，效率很低）：
SELECT
A.order_id,
A.wid,
A.work_name,
A.supply_price,
A.sell_price,
A.total_num,
A.sell_profit,
A.sell_percent,
A.goods_id,
A.goods_name,
A.classify,
B.gb_name
FROM
(
SELECT
A.sub_order_id AS order_id,
A.photo_id AS wid,
A.photo_name AS work_name,
A.supply_price,
A.sell_price,
sum(A.num) AS total_num,
(
A.sell_price - A.supply_price
) AS sell_profit,
(
A.sell_price - A.supply_price
) / A.sell_price AS sell_percent,
A.goods_id,
A.goods_name,
B.goods_name AS classify
FROM
order_goods AS A
LEFT JOIN (
SELECT
A.goods_id,
A.parentid,
B.goods_name
FROM
test_qyg_goods.goods AS A
LEFT JOIN test_qyg_goods.goods AS B ON A.parentid = B.goods_id
) AS B ON A.goods_id = B.goods_id
WHERE
A.createtime >= '2016-09-09 00:00:00'
AND A.createtime = '2016-10-16 23:59:59'
AND FROM_UNIXTIME(
UNIX_TIMESTAMP(A.createtime),
'%Y-%m-%d'
) != '2016-09-28'
AND FROM_UNIXTIME(
UNIX_TIMESTAMP(A.createtime),
'%Y-%m-%d'
) != '2016-10-07'
GROUP BY
A.photo_id
ORDER BY
A.goods_id ASC
) AS A
LEFT JOIN (
SELECT
A.wid,
A.brand_id,
B.gb_name
FROM
test_qyg_user.buser_goods_list AS A
LEFT JOIN test_qyg_supplier.brands AS B ON A.brand_id = B.gbid
) AS B ON A.wid = B.wid
查詢結果耗時4秒多，explain分析，發現其中2個子查詢是全部掃描，可以使用mysql的多個left join優化
SELECT
A.sub_order_id,
A.photo_id AS wid,
A.photo_name AS work_name,
A.supply_price,
A.sell_price,
sum(A.num) AS total_num,
(
A.sell_price - A.supply_price
) AS sell_profit,
(
A.sell_price - A.supply_price
) / A.sell_price AS sell_percent,
A.goods_id,
A.goods_name,
B.parentid,
C.goods_name AS classify,
D.brand_id,
E.gb_name,
sum(
CASE
WHEN F.buy_type = 'yes' THEN
A.num
ELSE
0
END
) AS total_buy_num,
sum(
CASE
WHEN F.buy_type = 'yes' THEN
A.num
ELSE
0
END * A.sell_price
) AS total_buy_money,
sum(
CASE
WHEN F.buy_type = 'no' THEN
A.num
ELSE
0
END
) AS total_give_num,
sum(
CASE
WHEN F.buy_type = 'no' THEN
A.num
ELSE
0
END * A.sell_price
) AS total_give_money
FROM
order_goods AS A
LEFT JOIN test_qyg_goods.goods AS B ON A.goods_id = B.goods_id
LEFT JOIN test_qyg_goods.goods AS C ON B.parentid = C.goods_id
LEFT JOIN test_qyg_user.buser_goods_list AS D ON A.photo_id = D.wid
LEFT JOIN test_qyg_supplier.brands AS E ON D.brand_id = E.gbid
LEFT JOIN order_info_sub AS F ON A.sub_order_id = F.order_id
WHERE
A.createtime >= '2016-09-09 00:00:00'
AND A.createtime = '2016-10-16 23:59:59'
AND FROM_UNIXTIME(
UNIX_TIMESTAMP(A.createtime),
'%Y-%m-%d'
) != '2016-09-28'
AND FROM_UNIXTIME(
UNIX_TIMESTAMP(A.createtime),
'%Y-%m-%d'
) != '2016-10-07'
GROUP BY
A.photo_id
ORDER BY
A.goods_id ASC
查詢結果耗時0.04秒
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