MySQL-从3个表中获取不同值的计数

Question

I have created 3 tables named complete_shifts,incomplete_shifts and incomplete_shift_register. All three tables include 2 columns called employee_no and date. I want to get count of distinct values in employee_no column from all three tables on a specified date.

Ex: if I specify " date = 2013-06-13 " then count shuld be 3 . (employee no : 0008,0019,0035 )

complete_shifts

shift_id |   date   | employee_no | sign_in_at | sign_out_at

---

1        |2013-06-13|   0008      | 7:05       | 16:05
2        |2013-06-13|   0008      | 7:10       | 16:05
3        |2013-06-14|   0025      | 7:11       | 16:10

incomplete_shifts

 shift_id  |    date   | employee_no |  sign_in_at |sign_out_at

---

   1       | 2013-06-13|    0019     |   7:08      |    

incomplete_shift_register

 shift_id |   date    |employee_no | sign_in_at

---

  1       | 2013-06-13|   0008     |    7:08
  2       | 2013-06-13|   0035     |    7:09
  3       | 2013-06-14|   0060     |    7:11

i don't know how to write the SQL syntax for above situation... plezzz help me..

thank you.

Answer 1

How about something like

SELECT  COUNT(DISTINCT employee_no) cnt_employee_no
FROM    (
    SELECT  employee_no
    FROM    complete_shifts
    WHERE   date = '2013-06-13'
    UNION 
    SELECT  employee_no
    FROM    incomplete_shifts
    WHERE   date = '2013-06-13'
    UNION 
    SELECT  employee_no
    FROM    incomplete_shift_register
    WHERE   date = '2013-06-13'
    )

Using

UNION Syntax

UNION is used to combine the result from multiple SELECT statements into a single result set.

and

COUNT(DISTINCT expr,[expr...])

Returns a count of the number of rows with different non-NULL expr values.

Answer 2

Just UNION the tables together and COUNT . The UNION will remove duplicates, so a simple COUNT will do the trick - a COUNT DISTINCT won't be needed. This will work if you want the count only; if you want other values the query gets slightly more involved:

SELECT COUNT(*) FROM (
  SELECT employee_no
    FROM complete_shifts
    WHERE date = '2013-06-13'
  UNION SELECT employee_no
    FROM incomplete_shifts
    WHERE date = '2013-06-13'
  UNION SELECT employee_no
    FROM incomplete_shift_register
    WHERE date = '2013-06-13'
 ) AllShifts

Answer 3

SELECT count(*) from 
(
SELECT 1 from complete_shifts WHERE date="2013-06-13"
UNION ALL
SELECT 1 from incomplete_shifts WHERE date="2013-06-13"
UNION ALL
SELECT 1 from incomplete_shift_register WHERE date="2013-06-13"
)t