MySQL：基于select语句的信息更新多个值

Question

I have on table were dates are mixed up, the startDate and endDate columns need to be switched. I am use to using SQL Server and this could be accomplished using a cursor in SQL Server, but in MySQL you can only use a cursor inside a SP which i do not want

The following statement returns all the records that need to be updated

select * from calendarTable where endDate < startDate;

I have tried the following code but this first sets the startDate to the value of the endDate then the endDate to the new value of the startDate so this ends as both dates being the same and not switched

update calendarTable
set startDate  = endDate,
    endDate = startDate
where startDate > endDate;

Is there a way of doing this in MySQL similar to a cursor in SQL Server or how would you do this in MySQL?

Answer 1

MySQL for the most part evaluates assignments in left-to-right order. If you want to swap the values in two fields, you have to use a temp variable to hold one of the values while moving things around:

UPDATE @temp := startDate, startDate = endDate, endDate = @temp

Assignments within a statement take immediate effect, so if you "reuse" a changed field later on in the query, you get the updated value, not the original one:

mysql> select * from foo;
+------+
| x    |
+------+
|    1 |
|   10 |
+------+
2 rows in set (0.00 sec)

mysql> update foo set x=x+1, x=x+50;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from foo;
+------+
| x    |
+------+
|   52 |
|   61 |
+------+
2 rows in set (0.00 sec)

if MySQL used the "original" values for the right-hand side, then the final table would've been 51 and 60 , because the values created by the +1 component would've been trashed by the later +10 .

Instead, you get a "live" value at each usage in the query, so a field's value depends entirely on what you're doing with it inside the query, and where you're using it in the query.

Answer 2

i managed to do this by using an joining on a select as below

UPDATE calendarTable as cal, 
(
    select id,startDate,endDate from calendarTable where endDate < startDate
) as temp
SET cal.startDate = temp.endDate, cal.endDate = temp.startDate  WHERE cal.ID = temp.ID;