我有以下数据库模式:
CREATE SCHEMA products; CREATE TABLE products.product_group ( id bigserial NOT NULL PRIMARY KEY,name varchar(255) NOT NULL ) CREATE TABLE products.product ( id bigserial NOT NULL PRIMARY KEY,name varchar(255) NOT NULL,product_group_id bigint REFERENCES products.product_group(id),color varchar(255) NOT NULL ) INSERT INTO products.product_group(name) values ('product group 1'); INSERT INTO products.product(name,product_group_id,color) VALUES ('product 1',1,'RED'); INSERT INTO products.product(name,'BLUE'); INSERT INTO products.product(name,'GREEN');
如果我执行以下查询:
SELECT count(*),pg.name FROM products.product p LEFT JOIN products.product_group pg ON (p.product_group_id=pg.id) WHERE pg.id = 1 AND p.color = 'RED' GROUP BY pg.name
我得到了结果:
1;"product group 1"
如果我执行以下查询:
SELECT count(*),pg.name FROM products.product p LEFT JOIN products.product_group pg ON (p.product_group_id=pg.id) WHERE pg.id = 1 AND p.color = 'YELLOW' GROUP BY pg.name
我没有得到任何结果,我想要:
0;"product group 1"
如何更改查询以获得我想要的结果?
解决方法
SELECT count(p.name),pg.name FROM products.product p right JOIN products.product_group pg ON p.product_group_id=pg.id and pg.id = 1 and p.color = 'YELLOW' GROUP BY pg.name; count | name -------+----------------- 0 | product group 1
您的原始查询有两个问题.首先,外部连接是向后的.其次,where条件将外连接转换为内连接,因此必须将where条件移动到连接条件.
计数还有另一个问题.您必须在计数中声明一列,以便它只计算该列不为空的位置.