MySQL选择表的第一行

Question

I know there are dozens of "select the first row" questions, but I have an interesting situation that I don't think I can fix with a LIMIT.

I'm trying to show properties that are available between a start and end date, with N days consecutively available. I'm so close.

I'm going to break this query apart into bits because it's a big one.

Blocked Dates (part 1)

(SELECT c.id,c.property_id,c.start_date,c.end_date FROM property_calendar as c WHERE c.start_date <= '.$endQuote.' AND c.end_date >= '.$startQuote)
UNION
(SELECT l.id,l.property_id,l.start_date,l.end_date FROM lease as l WHERE l.start_date <= '.$endQuote.' AND l.end_date >= '.$startQuote.' AND l.status < 6)
ORDER BY start_date ASC

This union smashes our two calendars together for easy comparison. Each entry has a start_date and end_date. Dates that fall between (including the start and end) are considered to be unavailable.

The Calculator (part 2)

SELECT IFNULL(DATEDIFF( IFNULL(ct2.start_date, '2015-12-09'), ct1.end_date) - 1, 0) as open_days,
(DATEDIFF(ct1.start_date, '2015-12-01') - 1) as first_open,
IFNULL(DATEDIFF( LEAST(ct1.end_date, '2015-12-09'), GREATEST(ct1.start_date, '2015-12-01' ) ) + 1, 0)  as blocked_days,
ct1.property_id as property_id
FROM ({blocked dates}) as ct1
LEFT JOIN ({blocked dates}) as ct2 ON ct2.property_id = ct1.property_id AND ct2.start_date > ct1.end_date
GROUP BY ct1.property_id,ct1.start_date

This takes the table in part 1, and joins it to itself allowing us to compare the previous blocked date with the next. open_days is the negative space between each blocked date. blocked_days is just how many days are blocked in ct1 . The part that's causing trouble is first_open .

This query works, unless the first blocked date occurs after the start date the user gave. In that case, we miss out on counting those first few days that are open. So, part of my solution was to add the first_open which just simply calculates DATEDIFF( ct1.start_date, {the given start date}) .

The Final Sum

SELECT cj.property_id, 
IFNULL(SUM(cj.blocked_days), 0) as total_blocked,
IFNULL(GREATEST(MAX(cj.open_days), MAX(cj.first_open)), 0) as consec_days_avail
FROM {the calculator} as cj

As I said, here is where I have the problem. Selecting MAX(cj.first_open) obviously won't work for me. There's no unique ID either I can go by because the availability is the combination of two tables where the IDs can possibly collide. And MySQL doesn't have a FIRST() function to use. But basically I only care about the greatest available consecutive days because we just don't care where the consecutive days occur, just that they happen between the user's start and end dates.

So is there a better way to structure this query? I've tried exploring methods to get only the first row in The Calculator to actually DATEDIFF() and all others are 0, but the problem I had using IF(ct1.start_date = MIN(ct1.start_date), DATEDIFF(...), 0) was that the first two rows would have a non 0 value and I only want just the first. And I think that has something to do with how I'm joining a table on itself.

Any help is appreciated. Thanks.

Edit

For the curious parties here's the query all in one piece. This is now working (as far as I can tell) as intended. Now I'm just wondering if there's a better way or if there's any tips.

SELECT p.* as id, IFNULL(cd.total_blocked, 0) as total_blocked, IFNULL(cd.consec_days_avail, 0) as consec_days_avail
FROM properties AS p
LEFT JOIN (
SELECT cj.property_id, IFNULL(SUM(cj.blocked_days), 0) as total_blocked, IFNULL(GREATEST(MAX(cj.open_days), MAX(cj.first_open)), 0) as consec_days_avail
FROM (
SELECT IFNULL(DATEDIFF( IFNULL(ct2.start_date2, {user_end_date}), ct1.end_date) - 1, 0) as open_days,IF( MIN(ct1.start_date) = ct1.start_date,  DATEDIFF(ct1.start_date, {user_start_date}), 0) as first_open,IFNULL(DATEDIFF( LEAST(ct1.end_date, {user_end_date}), GREATEST(ct1.start_date, {user_start_date} ) ) + 1, 0)  as blocked_days,ct1.property_id as property_id
FROM ((SELECT c.id,c.property_id,c.start_date,c.end_date FROM property_calendar as c WHERE c.start_date <= {user_end_date} AND c.end_date >= {user_start_date}) UNION (SELECT l.id,l.property_id,l.start_date,l.end_date FROM lease as l WHERE l.start_date <= {user_end_date} AND l.end_date >= {user_start_date} AND l.status < 6) ORDER BY start_date ASC) as ct1
LEFT JOIN ((SELECT c.id as id2,c.property_id as property_id2,c.start_date as start_date2,c.end_date as end_date2 FROM property_calendar as c WHERE c.start_date <= {user_end_date} AND c.end_date >= {user_start_date}) UNION (SELECT l.id,l.property_id,l.start_date,l.end_date FROM lease as l WHERE l.start_date <= {user_end_date} AND l.end_date >= {user_start_date} AND l.status < 6) ORDER BY start_date2 ASC) as ct2 ON ct2.property_id2 = ct1.property_id AND ct2.start_date2 > ct1.end_date
GROUP BY ct1.property_id,ct1.start_date) as cj
GROUP BY cj.property_id) as cd on cd.property_id = p.id
WHERE p.state = 1
GROUP BY p.id
HAVING total_blocked < DATEDIFF({user_end_date},{user_start_date}) + 1 AND ( consec_days_avail >= {user_days_avail} OR total_blocked = 0 )
ORDER BY p.id asc

Answer 1

Hello!
The solution for that problem is a SQL Function called MIN and it works like this:

SELECT MIN(primary_key_column),the_other_columns FROM table_name;

As you can see , at the start of the Query (After the word SELECT) you just Call the function MIN and give the column that also is the Primary Key of the table.
Because this function find the minimum of the registers of the table.I mean that if the selected column is an INTEGER it will be going to find the row with the Smallest Number in that column

If you are using PHP too there is a function to make that called mysql_fetch_array ,for this function you have to make first a quy with another PHP function called mysql_query and use it like this

$myQuery = mysql_query("SELECT * FROM tablename",$conectionVariable)or die(mysql_error);

Then we call the function mysql_fetch_array and store it in a variable like this:

$myQueryArray = mysql_getch_array($myQuery)or die(mysql_error());

Now for use it you have to call it like a normal Array:

$myQueryArray['COLUMN_NAME'] And it return you the value of that column is the first Row

Well,that all.

Thanks!