PostgreSQL: Flattening a relation with an array to emit one row per array entry

问题

Given a table defined as such:

CREATE TABLE test_values(name TEXT, values INTEGER[]);

...and the following values:

| name  | values  |
+-------+---------+
| hello | {1,2,3} |
| world | {4,5,6} |

I'm trying to find a query which will return:

| name  | value |
+-------+-------+
| hello | 1     |
| hello | 2     |
| hello | 3     |
| world | 4     |
| world | 5     |
| world | 6     |

I've reviewed the upstream documentation on accessing arrays, and tried to think about what a solution using the unnest() function would look like, but have been coming up empty.

An ideal solution would be easy to use even in cases where there were a significant number of columns other than the array being expanded and no primary key. Handling a case with more than one array is not important.

回答1:

You can put the set-returning function unnest() into the SELECT list like Raphaël suggests. But in Postgres 9.3 or later use a LATERAL join for this instead. It is the cleaner, preferable, standard-compliant way to put set-returning functions into the FROM list, not into the SELECT list:

SELECT name, value
FROM   tbl, unnest(values) value;  -- implicit CROSS JOIN LATERAL

One subtle difference: this drops rows with empty / NULL values from the result since unnest() returns no row, while the same is converted to a NULL value in the FROM list and returned anyway. The 100 % equivalent query is:

SELECT t.name, v.value
FROM   tbl t
LEFT   JOIN unnest(t.values) v(value) ON true;

What is the difference between LATERAL and a subquery in PostgreSQL?

回答2:

Well, you give the data, the doc, so... let's mix it ;)

select 
 name, 
 unnest(values) as value 
from test_values

see SqlFiddle

来源：https://stackoverflow.com/questions/31997662/postgresql-flattening-a-relation-with-an-array-to-emit-one-row-per-array-entry

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arrays

postgresql

set-returning-functions

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