如何解决 MySQL 中的 null 值？

Question

我想在 MySQL 中使用相同的查询将数据与收入和支出表分开显示，但它会在显示收入标题日期时显示，然后显示相同的显示支出标题日期。但是如果它找到收入表的第 4 行，如果找到费用表的第 2 行，我需要这样做，但它显示了第 4 行。为什么？

SELECT  income.date, 
        IF(income.income_sourch IS NULL, 'N/A', income.income_sourch )as 'Income Title',
        income.amount as 'income',
        IF(expense.expense_sourche IS NULL, 'N/A', expense.expense_sourche)  as 'ExpenseTitle',
        expense.amount as 'Expense'
from expense  RIGHT JOIN income
     ON expense.date=income.date
where expense.date='30-Aug-2016'

Answer 1

为什么 - 因为当您只有一个日期来连接 2 个表时，这就是连接的工作方式。要解决此问题，您需要创建除日期之外的关系，记住在任何时候收入记录的数量可能大于、小于或等于费用记录的数量。如果我们为收入中的每一行生成一个行号，并为支出中的每一行独立生成一个行号，那么我们可以在行号上加入收入和支出 例如

/*
CREATE TABLE INCOME (ID INT,DESCRIPTION VARCHAR(10),DT DATE,AMOUNT INT);
INSERT INTO INCOME VALUES
(1,'SALE','2016-08-30',100),
(2,'ROKY','2016-08-30',200),
(3,'KORIM','2016-08-30',300),
(4,'SALEVOUCHR','2016-08-30',400);
CREATE TABLE EXPENSES (ID INT,DESCRIPTION VARCHAR(10),DT DATE,AMOUNT INT);
TRUNCATE TABLE EXPENSES;
INSERT INTO EXPENSES VALUES
(1,'RENT','2016-08-30',100),
(2,'UTILITIES','2016-08-30',100);
/*

这段代码

SELECT S.DT,S.DESCRIPTION,S.AMOUNT, s.rn,T.DESCRIPTION,T.AMOUNT, t.rn1
FROM
(
SELECT I.DT,I.DESCRIPTION,I.AMOUNT,
            @RN:=@RN + 1 RN
FROM     (SELECT @RN:=0) RN,INCOME I
) S 
LEFT OUTER JOIN
(SELECT E.DT,E.DESCRIPTION,E.AMOUNT,
            @RN1:=@RN1 + 1 RN1
FROM     (SELECT @RN1:=0) RN1, EXPENSES E
) T ON T.RN1 = S.RN AND T.DT = S.DT
WHERE S.DT IS NOT NULL 

结果

+------------+-------------+--------+------+-------------+--------+------+
| DT         | DESCRIPTION | AMOUNT | rn   | DESCRIPTION | AMOUNT | rn1  |
+------------+-------------+--------+------+-------------+--------+------+
| 2016-08-30 | SALE        |    100 |    1 | RENT        |    100 |    1 |
| 2016-08-30 | ROKY        |    200 |    2 | UTILITIES   |    100 |    2 |
| 2016-08-30 | KORIM       |    300 |    3 | NULL        |   NULL | NULL |
| 2016-08-30 | SALEVOUCHR  |    400 |    4 | NULL        |   NULL | NULL |
+------------+-------------+--------+------+-------------+--------+------+

但这是不安全的，因为它假定收入总是多于支出项目 例如，如果我们再添加 3 个费用行

TRUNCATE TABLE EXPENSES;
INSERT INTO EXPENSES VALUES
(1,'RENT','2016-08-30',100),
(2,'UTILITIES','2016-08-30',100),
(3,'RATES','2016-08-30',100),(4,'SALARY','2016-08-30',100),(5,'INSURANCE','2016-08-30',100);

那么最后一个费用项目丢失导致

+------------+-------------+--------+------+-------------+--------+------+
| DT         | DESCRIPTION | AMOUNT | rn   | DESCRIPTION | AMOUNT | rn1  |
+------------+-------------+--------+------+-------------+--------+------+
| 2016-08-30 | SALE        |    100 |    1 | RENT        |    100 |    1 |
| 2016-08-30 | ROKY        |    200 |    2 | UTILITIES   |    100 |    2 |
| 2016-08-30 | KORIM       |    300 |    3 | RENT        |    100 |    3 |
| 2016-08-30 | SALEVOUCHR  |    400 |    4 | UTILITIES   |    100 |    4 |
+------------+-------------+--------+------+-------------+--------+------+

为了解决这个问题，我们可以像这样模拟完全连接

SELECT S.*,T.*
FROM
(
SELECT I.DT,I.DESCRIPTION,I.AMOUNT,
            @RN:=@RN + 1 RN
FROM     (SELECT @RN:=0) RN,INCOME I
) S 
LEFT JOIN
(SELECT E.DT,E.DESCRIPTION,E.AMOUNT,
            @RN1:=@RN1 + 1 RN1
FROM     (SELECT @RN1:=0) RN1, EXPENSES E
) T ON T.RN1 = S.RN AND T.DT = S.DT

UNION

SELECT S.*,T.*
FROM
(
SELECT I.DT,I.DESCRIPTION,I.AMOUNT,
            @RN3:=@RN3 + 1 RN3
FROM     (SELECT @RN3:=0) RN3,INCOME I
) S 
RIGHT JOIN
(SELECT E.DT,E.DESCRIPTION,E.AMOUNT,
            @RN4:=@RN4 + 1 RN4
FROM     (SELECT @RN4:=0) RN4, EXPENSES E
) T ON T.RN4 = S.RN3 AND T.DT = S.DT

结果

+------------+-------------+--------+------+------------+-------------+--------+------+
| DT         | DESCRIPTION | AMOUNT | RN   | DT         | DESCRIPTION | AMOUNT | RN1  |
+------------+-------------+--------+------+------------+-------------+--------+------+
| 2016-08-30 | SALE        |    100 |    1 | 2016-08-30 | RENT        |    100 |    1 |
| 2016-08-30 | ROKY        |    200 |    2 | 2016-08-30 | UTILITIES   |    100 |    2 |
| 2016-08-30 | KORIM       |    300 |    3 | 2016-08-30 | RATES       |    100 |    3 |
| 2016-08-30 | SALEVOUCHR  |    400 |    4 | 2016-08-30 | SALARY      |    100 |    4 |
| NULL       | NULL        |   NULL | NULL | 2016-08-30 | INSURANCE   |    100 |    5 |
+------------+-------------+--------+------+------------+-------------+--------+------+

Answer 2

您只需要在查询中添加限制

如果费用表限制为 2 if 其他限制 4

查看如何使用 if 条件为查询添加限制