PostgreSQL 实现子查询返回多行的案例

2022-03-31 09:44 浏览：342

作为一个表达式使用的子查询返回了多列：

在查询中，我们需要以第2条查询语句作为第一条查询语句的条件，但是第一条根据这个条件查询出来是多个数据，这时候又需要保留多个数据，运用子查询就会报错，

以下就为解决这种多对多关系查询，且没有关联关系的表的解决方案：

select c.rain_value,c.ad_nm from
(
select *, json::json->t2.lon_lat as rain_value from actual_time_model_json t1,
(SELECT DISTINCT lon || '_' || lat as lon_lat,ad_nm from grid_all_points_null)t2 where section='0' and t1.filename = 'Z_NWGD_C_BCCD_20180711022009_P_RFFC_SPCC-ER01_201807110800_02401.GRB2'
)c where c.rain_value is not null

补充：PostgreSQL 的子查询相关的知识，exists,any,all

Subquery

SELECT
film_id,
title,
rental_rate
FROM
film
WHERE
rental_rate > (
SELECT
AVG (rental_rate)
FROM
film
);

有了子查询，在设定需要查询表才能得到查询条件时，就可以直接在一条语句中写，不用分开多条写了，方便了许多。

子查询返回多条时，可以在 where 子句中用 IN，来匹配查询条件。

SELECT
film_id,
title
FROM
film
WHERE
film_id IN (
SELECT
   inventory.film_id
FROM
   rental
INNER JOIN inventory ON inventory.inventory_id = rental.inventory_id
WHERE
   return_date BETWEEN '2005-05-29'
AND '2005-05-30'
);

EXISTS 操作符

在 where 子句的查询条件中，exists 操作符，会在子查询有返回行时，返回true；不论返回几行。

因此，子查询中的查询字段仅写1就好；标准的写法：EXISTS (SELECT 1 FROM tbl WHERE condition)

SELECT
first_name,
last_name
FROM
customer
WHERE
EXISTS (
SELECT
   1
FROM
   payment
WHERE
   payment.customer_id = customer.customer_id
);

NO EXISTS ,与之相反，当子查询返回0行时，返回true

SELECT first_name,
last_name
FROM customer c
WHERE NOT EXISTS
(SELECT 1
FROM payment p
WHERE p.customer_id = c.customer_id
AND amount > 11 )
ORDER BY first_name,
last_name;

当子查询返回 NULL，会返回true, 也就是返回所有行。

SELECT
first_name,
last_name
FROM
customer
WHERE
EXISTS( SELECT NULL )
ORDER BY
first_name,
last_name;

ANY

与任何子查询返回的值匹配就返回 true

expresion operator ANY(subquery)

表达式一般为字段

操作符为 >,<,=,<>,>=,<=

ANY 可以与 SOME 替换

子查询返回的必须是一列，

SELECT title
FROM film
WHERE length >= ANY(
SELECT MAX( length )
FROM film
INNER JOIN film_category USING(film_id)
GROUP BY category_id );

The = ANY is equivalent to IN operator.

Note that the <> ANY operator is different from NOT IN. The following expression:

1x <> ANY (a,b,c)

is equivalent to

1x <> a OR x <> b OR x <> c

ALL

所有子查询返回的值匹配则返回 true

也就是大于最大，小于最小

SELECT
film_id,
title,
length
FROM
film
WHERE
length > ALL (
   SELECT
    ROUND(AVG (length),2)
   FROM
    film
   GROUP BY
    rating
)
ORDER BY
length;