无法弄清楚如何编写此查询

Question

I have a couple hundred records in a table and I need to get the the last 50 of them, but I need 25 males and 25 females. There is a gender column. I just can't figure out how to make that query. Can I get some help?

Answer 1

If you need exactly 50, use a UNION which gets the last 25 of each. This assumes you have some auto-incrementing id column which is sorted in descending order to retrieve the newest 25 of each gender.

In order to use an ORDER BY and LIMIT for individual components of a UNION , MySQL expects those components to be enclosed in () .

(SELECT * FROM tbl WHERE gender='F' ORDER BY id DESC LIMIT 25)
UNION ALL
(SELECT * FROM tbl WHERE gender='M' ORDER BY id DESC LIMIT 25)

Note : By the way, I've used SELECT * here for brevity but you should never actually do that in a UNION query. Instead, explicitly list the columns you need, and make sure they're in the same order in both parts of the UNION .

Like:

(SELECT col1, col2, coln FROM tbl WHERE gender='F'....)
UNION ALL
(SELECT col1, col2, coln FROM tbl WHERE gender='M'....)

Answer 2

(SELECT * FROM table WHERE gender = 'M' ORDER BY id DESC LIMIT 25)
UNION
(SELECT * FROM table WHERE gender = 'F' ORDER BY id DESC LIMIT 25)

That would work I guess. For the order by ID part I assumed you have an autoincrement id column, making the last added records the ones with the highest ID.

Answer 3

I just thought of this quickly, no guarantees.

SELECT * FROM table WHERE gender = 'male' ORDER BY ID DESC LIMIT 25
UNION ALL
SELECT * FROM table WHERE gender = 'female' ORDER BY ID DESC LIMIT 25

Basically, it gets the last 25 men, and the last 25 women, and combines the results together.

Answer 4

there is no such concept of LAST unless you have another column which you can use to ORDER BY .

i suggest write 2 queries, on for males only, and another for females only.

then use an order by so there is a 'last' set.

then flip that (using DESC ) so they are FIRST instead.

then use LIMIT clause to select the number you want.

then finally, UNION them together.

Answer 5

Try using two queries, one to grab the 25 latest males and a second to grab the 25 latest females. Then just concatenate the results.

SELECT * FROM males LIMIT 25 ORDER BY DESC

SELECT * FROM females LIMIT 25 ORDER BY DESC