MySQL-重用嵌套的select语句

Question

I'm wondering whether there's a more efficient way of writing the following query (note the reuse of the same select statement resulting in A). I wouldn't worry too much about it, but I need to add one or two more 'parent' levels to this, and it just seems like there must be a better way than my naive approach.

SELECT A.*, I.INVOICE_STATUS
FROM ( SELECT E.*, IL.INVOICE_LINE_ID, IL.INVOICE_LINE_INVOICE_ID
       FROM EXPENSE E
       LEFT JOIN INVOICE_LINE IL
       ON E.EXPENSE_ID = IL.INVOICE_LINE_EXPENSE_ID
       UNION
       SELECT E.*, IL.INVOICE_LINE_ID, IL.INVOICE_LINE_INVOICE_ID
       FROM EXPENSE E
       RIGHT JOIN INVOICE_LINE IL
       ON E.EXPENSE_ID = IL.INVOICE_LINE_EXPENSE_ID) A
LEFT JOIN INVOICE I
ON A.INVOICE_LINE_INVOICE_ID = I.INVOICE_ID
UNION
SELECT A.*, I.INVOICE_STATUS, I.INVOICE_DUE_DATE
FROM ( SELECT E.*, IL.INVOICE_LINE_ID, IL.INVOICE_LINE_INVOICE_ID
       FROM EXPENSE E
       LEFT JOIN INVOICE_LINE IL
       ON E.EXPENSE_ID = IL.INVOICE_LINE_EXPENSE_ID
       UNION
       SELECT E.*, IL.INVOICE_LINE_ID, IL.INVOICE_LINE_INVOICE_ID
       FROM EXPENSE E
       RIGHT JOIN INVOICE_LINE IL
       ON E.EXPENSE_ID = IL.INVOICE_LINE_EXPENSE_ID) A
RIGHT JOIN INVOICE I
ON A.INVOICE_LINE_INVOICE_ID = I.INVOICE_ID;

Thanks!

Answer 1

In Oracle, you could create what is called a common table expression with the WITH keyword. It works as follows:

WITH ORACLE_CTE AS (SELECT * FROM DUAL CONNECT BY LEVEL <= 10);

In MySQL, you don't have such luxuries as you do in Oracle. So, you would do as follows:

Create a procedure to create a temporary table that stores the result set of the following:

SELECT E.*, IL.INVOICE_LINE_ID, IL.INVOICE_LINE_INVOICE_ID
FROM EXPENSE E
LEFT JOIN INVOICE_LINE IL
ON E.EXPENSE_ID = IL.INVOICE_LINE_EXPENSE_ID
UNION
SELECT E.*, IL.INVOICE_LINE_ID, IL.INVOICE_LINE_INVOICE_ID
FROM EXPENSE E
RIGHT JOIN INVOICE_LINE IL
ON E.EXPENSE_ID = IL.INVOICE_LINE_EXPENSE_ID;

And then, reuse the temporary table in both of your queries across which you are applying a UNION.

SELECT A.*, I.INVOICE_STATUS
FROM CTE_MYSQL_TABLE
LEFT JOIN INVOICE I
ON A.INVOICE_LINE_INVOICE_ID = I.INVOICE_ID
UNION
SELECT A.*, I.INVOICE_STATUS, I.INVOICE_DUE_DATE
FROM CTE_MYSQL_TABLE
RIGHT JOIN INVOICE I
ON A.INVOICE_LINE_INVOICE_ID = I.INVOICE_ID;