PostgreSQL外连接查询从一个表中获取所有数据

Question

我有两张桌子

CREATE TABLE REFERENCE_VALUES
(
REFERENCE_ID SERIAL,
REFERENCE_OBJ_NAME TEXT,
REFERENCE_VALUE_CODE BIGINT,
DISPLAY_NAME TEXT,
CREATED_ON TIMESTAMP,
MODIFIED_ON TIMESTAMP
);

ALTER TABLE REFERENCE_VALUES
ADD PRIMARY KEY (ID);

ALTER TABLE REFERENCE_VALUES
ADD FOREIGN KEY (REFERENCE_VALUE_CODE)
REFERENCES CATEGORY(ID);

CREATE TABLE CATEGORY
(
ID BIGSERIAL NOT NULL,
CODE TEXT NOT NULL,
NAME TEXT NOT NULL,
PARENT_ID BIGINT,
PATH TEXT,
COMP_ID BIGINT,
CREATED_ON TIMESTAMP,
MODIFIED_ON TIMESTAMP,
);

ALTER TABLE CATEGORY
ADD PRIMARY KEY (ID);

ALTER TABLE CATEGORY
ADD FOREIGN KEY (COMP_ID)
REFERENCES COMPANY_ID(ID);

我在REFERENCE_VALUES表中的显示名称为DISPLAY_NAME，我想从表CATEGORY中选择所有值，但是NAME的值（在CATEGORY表中）由REFERENCE_VALUES中的DISPLAY_NAME的值替换，并且如果DISPLAY_NAME的值为NULL或EMPTY，我会保留NAME（CATEGORY表）的值。

我已经能够通过下面的查询做到这一点

SELECT C.ID, C.CODE, COALESCE(R.DISPLAY_NAME, C.NAME) as NAME, C.PARENT_ID, C.PATH, C.COMP_ID, C.CREATED_ON, C.MODIFIED_ON 
FROM CATEGORY C
LEFT JOIN REFERENCE_VALUES R
ON C.ID = R.REFERENCE_VALUE_CODE
WHERE R.REFERENCE_OBJ_NAME = 'CATEGORY';

但我只得到两个记录。 如何从类别表中获取所有记录？

样本数据以填充表格

INSERT INTO CATEGORY VALUES (1, 'CVB', 'COMM VEH', NULL, 'CVB', 1, '2016-05-13 15:50:19.985', NULL);
INSERT INTO CATEGORY VALUES (2, 'LVB', 'AUTO', NULL, 'LVB', 1, '2016-05-13 15:50:19.994', NULL);
INSERT INTO CATEGORY VALUES (3, 'INB', 'INF', NULL, 'INB', 1, '2016-05-13 15:50:19.997', NULL);
INSERT INTO CATEGORY VALUES (4, 'OHB', 'OFF', NULL, 'OHB', 1, '2016-05-13 15:50:20', NULL);
INSERT INTO CATEGORY VALUES (5, 'LUB', 'LUB', NULL, 'LUB', 1, '2016-05-13 15:50:20.002', NULL);
INSERT INTO CATEGORY VALUES (52, 'TRA', 'TIE', 32, 'CVB.HA.TRA', 1, '2016-05-13 15:51:32.605', NULL);
INSERT INTO CATEGORY VALUES (68, 'PF', 'PER', 42, 'LVB.LA.PF', 1, '2016-05-13 15:51:33.117', NULL);
INSERT INTO CATEGORY VALUES (73, 'CE', 'CAR', 32, 'CVB.HA.CE', 1, '2016-05-13 15:51:33.733', NULL);
INSERT INTO CATEGORY VALUES (74, 'KP', 'KP', 32, 'CVB.HA.KP', 1, '2016-05-13 15:51:33.958', NULL);
INSERT INTO CATEGORY VALUES (26, 'RP', 'RING', 11, 'OHB.OH.RP', 1, '2016-05-13 15:51:30.149', NULL);
INSERT INTO CATEGORY VALUES (47, 'CP', 'COMP', 9, 'CVB.CV.CP', 1, '2016-05-13 15:51:31.903', NULL);
INSERT INTO CATEGORY VALUES (48, 'TB', 'TUB', 9, 'CVB.CV.TB', 1, '2016-05-13 15:51:31.905', NULL);
INSERT INTO CATEGORY VALUES (18, 'FB', 'FILT', 11, 'OHB.OH.FB', 1, '2016-05-13 15:51:30.002', NULL);

INSERT INTO REFERENCE_VALUES (ID, REFERENCE_OBJ_NAME, REFERENCE_VALUE_CODE, DISPLAY_NAME) VALUES (1, 'CATEGORY', 6, INDU CHANGED);
INSERT INTO REFERENCE_VALUES (ID, REFERENCE_OBJ_NAME, REFERENCE_VALUE_CODE) VALUES (2, 'CATEGORY', 7);

Answer 1

WHERE条件过滤掉R.REFERENCE_OBJ_NAME为null的行，这是没有匹配的REFERENCE_VALUE的所有行。

也许您只是想加入REFERENCE_OBJ_NAME为“ CATEGORY”的REFERENCE_VALUES。 如果是这样，您可以这样做：

SELECT C.ID, C.CODE, COALESCE(R.DISPLAY_NAME, C.NAME) as NAME, C.PARENT_ID, C.PATH, C.COMP_ID, C.CREATED_ON, C.MODIFIED_ON 
FROM CATEGORY C
LEFT JOIN REFERENCE_VALUES R
ON R.REFERENCE_OBJ_NAME = 'CATEGORY' AND C.ID = R.REFERENCE_VALUE_CODE