左外連接不顯示在Postgres的

我有兩個表一個用於存儲蒙的日期和其他保持員工左外連接不顯示在Postgres的

在我的條件attendance_days表有31條記錄在員工考勤表考勤細節預期的結果一個僱員具有29和另一僱員8條共37個記錄數據如下

create table attendance_days 
(id serial primary key not null, 
attendance_date date); 



INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-01'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-02'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-03'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-04'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-05'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-06'); 

INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-07'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-08'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-09'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-10'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-11'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-12'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-13'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-14'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-15'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-16'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-17'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-18'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-19'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-20'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-21'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-22'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-23'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-24'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-25'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-26'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-27'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-28'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-29'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-30'); 
INSERT INTO attendance_days (attendance_date) VALUES ('2017-08-31');`

create table employee_attendance 
(id serial primary key not null, 
attendance_day_id integer , 
employee_id integer, 
first_in_time timestamp, 
last_out_time timestamp); 


INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (1, 407, '2017-08-01 09:30:00', '2017-08-01 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (2, 407, '2017-08-02 09:30:00', '2017-08-02 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (3, 407, '2017-08-03 09:30:00', '2017-08-03 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (4, 407, '2017-08-04 09:30:00', '2017-08-04 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (5, 407, '2017-08-05 09:30:00', '2017-08-05 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (6, 407, '2017-08-06 09:30:00', '2017-08-06 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (7, 407, '2017-08-07 09:30:00', '2017-08-07 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (8, 407, '2017-08-08 09:30:00', '2017-08-08 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (9, 407, '2017-08-09 09:30:00', '2017-08-09 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (12, 407, '2017-08-12 09:30:00', '2017-08-12 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (13, 407, '2017-08-13 09:30:00', '2017-08-13 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (14, 407, '2017-08-14 09:30:00', '2017-08-14 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (15, 601, '2017-08-15 10:44:40', '2017-08-15 10:47:36'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (15, 407, '2017-08-15 09:30:00', '2017-08-15 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (16, 407, '2017-08-16 09:30:00', '2017-08-16 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (17, 407, '2017-08-17 09:30:00', '2017-08-17 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (18, 407, '2017-08-18 09:30:00', '2017-08-18 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (18, 601, '2017-08-18 08:12:41', '2017-08-19 07:57:12'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (19, 601, '2017-08-19 07:57:12', '2017-08-19 20:14:01'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (19, 407, '2017-08-19 09:30:00', '2017-08-19 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (20, 407, '2017-08-20 09:30:00', '2017-08-20 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (21, 407, '2017-08-21 09:30:00', '2017-08-21 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (22, 601, '2017-08-22 08:07:00', '2017-08-22 20:43:10'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (22, 407, '2017-08-22 09:30:00', '2017-08-22 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (23, 407, '2017-08-23 09:30:00', '2017-08-23 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (24, 407, '2017-08-24 09:30:00', '2017-08-24 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (24, 601, '2017-08-24 08:06:58', '2017-08-24 20:03:44'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (25, 407, '2017-08-25 09:30:00', '2017-08-25 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (26, 601, '2017-08-26 09:14:24', '2017-08-26 20:21:32'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (26, 407, '2017-08-26 09:30:00', '2017-08-26 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (27, 407, '2017-08-27 09:30:00', '2017-08-27 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (27, 601, '2017-08-27 08:48:09', '2017-08-28 07:55:24'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (28, 407, '2017-08-28 09:30:00', '2017-08-28 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (29, 407, '2017-08-29 09:30:00', '2017-08-29 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (29, 601, '2017-08-29 08:09:44', '2017-08-29 20:04:26'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (30, 407, '2017-08-30 09:30:00', '2017-08-30 20:00:00'); 
INSERT INTO employee_attendance(attendance_day_id, employee_id, first_in_time, last_out_time) VALUES (31, 407, '2017-08-31 09:30:00', '2017-08-31 20:00:00');`

第一表格有31個記錄第二表（在08月31日）有37條記錄（29個記錄EMPLOYEE_ID 407和8記錄EMPLOYEE_ID 601

我執行以下查詢

SELECT * 
FROM attendance_days 
    LEFT OUTER JOIN employee_attendance 
       ON employee_attendance.attendance_day_id = attendance_days.id

它只返回37行，但我預計62行（每個員工31行）

我無法理解的問題

來源

2017-09-21 Pradeep Kumar P.C

你能解釋爲什麼你期望62行？ –

在第一個表格中有31行和第二個表格，第二個表格有兩個僱員的記錄 –

的Postgres找到匹配的行爲attendance_days的每個ID，所以沒有理由去創造artificia l命中（它看起來像「id，attendance_date，null，null，null，null，null」）。

您實際上期待得到像「id，attendance_date，null，null，employee_id，null，null」的結果。

所以你必須建立一個笛卡爾的attendance_days.id和employee_id產品（來自「employee table」），它給你兩個（62行）的每個組合。此笛卡爾產品可以與employee_attendance表外部連接以產生所需的結果。

來源

2017-09-21 07:47:52 Ronald

但是，如果我從employee_attendance表中刪除employee_id 407的記錄，它將顯示31條記錄 –

是的，具有匹配id的6行和25行與id不匹配。您期待的結果集包含每個employee_id的attendance_days完整列表以及可選的開始和結束時間這意味着每行至少應顯示：attendance_day.id，attendance_date _and_ employee_id。 – Ronald

是的，需要爲所有員工提供31天的結果 –

左外連接不顯示在Postgres的

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