[英]How to set condition in SQL query
SELECT partnumber, p.partdescription, i.invoicenumber, id.quantity, p.interchangeno, p.makemodelname , c.rte
FROM Invoice i,
invoicedetails id,
Parts p,
customer c
WHERE i.customerid = c.customerid
AND i.invoicenumber = id.invoicenumber
AND i.invoicenumber
AND id.partnumber = p.partNo
AND p.UnitsInStock < 0 AND id.quantity > 0;
In this query how to set condition that if interchangeNo is empty then above is right if interchangeNo is not empty then select interchangeNo as partNo在此查询中如何设置条件,如果interchangeNo 为空,则上面是正确的,如果interchangeNo 不为空,则选择interchangeNo 作为partNo
I guess you need a case statement -我想你需要一个案例陈述 -
SELECT partnumber, p.partdescription, i.invoicenumber, id.quantity,
CASE WHEN p.interchangeno IS NULL THEN p.interchangeno
ELSE partNo END interchangeno, p.makemodelname , c.rte
FROM Invoice i
JOIN invoicedetails id ON i.invoicenumber = id.invoicenumber
JOIN Parts p ON id.partnumber = p.partNo
JOIN customer c ON i.customerid = c.customerid
WHERE p.UnitsInStock < 0 AND id.quantity > 0;
have a look at https://www.w3schools.com/sql/func_mysql_if.asp and http://www.sqlservertutorial.net/sql-server-stored-procedures/sql-server-if-else/看看https://www.w3schools.com/sql/func_mysql_if.asp和http://www.sqlservertutorial.net/sql-server-stored-procedures/sql-server-if-else/
SELECT p.interchangeno
FROM Invoice i,
invoicedetails id,
Parts p,
customer c
WHERE i.customerid = c.customerid
AND i.invoicenumber = id.invoicenumber
AND i.invoicenumber
AND id.partnumber = p.partNo
AND p.UnitsInStock < 0 AND id.quantity > 0;
if (p.interchangeno =0)
SELECT partnumber, p.partdescription, i.invoicenumber, id.quantity, p.interchangeno, p.makemodelname , c.rte
FROM Invoice i,
invoicedetails id,
Parts p,
customer c
WHERE i.customerid = c.customerid
AND i.invoicenumber = id.invoicenumber
AND i.invoicenumber
AND id.partnumber = p.partNo
AND p.UnitsInStock < 0 AND id.quantity > 0;
else
-- code that you want
I'm pretty sure that this can be improved, but what you're missing is the if else我很确定这可以改进,但是您缺少的是 if else
You can use JOIN
query for connecting multiple tables, there is lot of examples for learning that.您可以使用
JOIN
查询连接多个表,有很多示例可供学习。
Your query will be like below if you have written query using JOIN
:如果您使用
JOIN
编写查询,您的查询将如下所示:
SELECT `INV_DTL`.`partnumber`, `P`.`partdescription`, `INV`.`invoicenumber`, `INV_DTL`.`quantity`,
(CASE WHEN `P`.`interchangeno` IS NULL THEN interchangeno_is_null ELSE interchangeno_is_not_null END) AS interchangeno, `P`.`makemodelname`, `C`.`rte`
FROM `invoice` INV
JOIN `invoicedetails` INV_DTL ON (`INV_DTL`.`invoicenumber` = `INV`.`invoicenumber`)
JOIN `parts` P ON (`P`.`partNo` = `INV_DTL`.`partnumber`)
JOIN `customer` C ON (`C`.`customerid` = `INV`.`customerid`)
WHERE `P`.`UnitsInStock` < 0 AND `INV_DTL`.`quantity` > 0;
A helpful link to learn JOIN
query.学习
JOIN
查询的有用链接。
You only need below query part for filling your issue:您只需要以下查询部分来填写您的问题:
CASE WHEN `P`.`interchangeno` IS NULL THEN interchangeno_is_null ELSE interchangeno_is_not_null END
Please use the required value on interchangeno_is_null
& interchangeno_is_not_null
.请在
interchangeno_is_null
和interchangeno_is_not_null
上使用所需的值。
The CASE
statement goes through conditions and returns a value when the first condition is met (like an IF-THEN-ELSE
statement). CASE
语句遍历条件并在满足第一个条件时返回一个值(如IF-THEN-ELSE
语句)。 So, once a condition is true, it will stop reading and return the result.因此,一旦条件为真,它将停止读取并返回结果。 If no conditions are true, it returns the value in the
ELSE
clause.如果没有条件为真,则返回
ELSE
子句中的值。
The syntax is like:语法是这样的:
CASE
WHEN condition1 THEN result1
WHEN condition2 THEN result2
WHEN conditionN THEN resultN
ELSE result
END;
You can also use IFNULL()
.您也可以使用
IFNULL()
。
SELECT partnumber, p.partdescription, i.invoicenumber, id.quantity,
IFNULL(p.interchangeno,p.partnumber), p.makemodelname , c.rte
FROM Invoice i join invoicedetails id on i.invoicenumber = id.invoicenumber
join Parts p on .... -- join condition goes here
join customer c on ... --join condition goes here
WHERE p.UnitsInStock < 0 AND id.quantity > 0
As mentioned above,always use modern, explicit JOIN
syntax as shown in my query如上所述,始终使用现代的、显式的
JOIN
语法,如我的查询所示
It's best to use the JOIN syntax.最好使用 JOIN 语法。
The comma syntax was already outdated by the end of the last century.到上世纪末,逗号语法已经过时了。
And then you can use LEFT JOIN
And a CASE
allows a bit of logic.然后你可以使用
LEFT JOIN
并且一个CASE
允许一些逻辑。
SELECT
CASE
WHEN p.partNo IS NULL
THEN id.partnumber
WHEN p.interchangeno IS NOT NULL
THEN id.partnumber
ELSE p.partNo
END AS partNo,
p.partdescription,
i.invoicenumber,
id.quantity,
p.interchangeno,
p.makemodelname,
c.rte
FROM Invoice i
INNER JOIN invoicedetails id
ON id.invoicenumber = i.invoicenumber
INNER JOIN customer c
ON c.customerid = i.customerid
LEFT JOIN Parts p
ON p.partNo = id.partnumber
AND p.UnitsInStock < 0
WHERE id.quantity > 0;
Use COALESCE()
(or its MySQL version IFNULL()
), something like this:使用
COALESCE()
(或其 MySQL 版本IFNULL()
),如下所示:
SELECT partnumber, p.partdescription, i.invoicenumber, id.quantity, p.interchangeno,
p.makemodelname, c.rte
FROM Invoice i
JOIN invoicedetails id ON i.invoicenumber = id.invoicenumber
JOIN Parts p ON id.partnumber = COALESCE(p.interchangeno, p.partNo)
JOIN customer c ON i.customerid = c.customerid
WHERE p.UnitsInStock < 0
AND id.quantity > 0
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