如何修复由knex中的union构建的查询

Question

I am using knex with pg for getting some data from my DB. The query that I am trying to build by knex is:

select "fixtime" from "positions" order by "fixtime" desc limit(1) 
union 
select "fixtime" from "positions" order by "fixtime" limit (1)

but, knex return the following query when I used 'union'. And I get the error when I am trying to get the result.

console.log(db.select('fixtime').from('positions').orderBy('fixtime').limit(1).union([db.select('fixtime').from('positions').orderBy('fixtime','desc').limit(1)]).toSQL())

this is the result of console:

select "fixtime" from "positions" 
union 
select "fixtime" from "positions" order by "fixtime" desc limit ? order by "fixtime" asc limit ?

db.select('fixtime').from('positions').orderBy('fixtime').limit(1).union([db.select('fixtime').from('positions').orderBy('fixtime','desc').limit(1)]).then(arr => console.log)

this is the error I got: Unhandled rejection error: syntax error at or near "order"

When I used a single query I can get the result. How can I fix this query with knex or is it a bug?

Answer 1

I believe this is not possible with Knex in the way you are trying it to do. There is an issue that describes something similar.

But... You can cheat your way to the result with Postgres by using two CTEs

const sql = db
  .with('first', qb => {
    qb.select('fixtime')
      .from('tc_positions')
      .orderBy('fixtime', 'asc')
      .limit(1);
  })
  .with('last', qb => {
    qb.select('fixtime')
      .from('tc_positions')
      .orderBy('fixtime', 'desc')
      .limit(1);
  })
  .select('*')
  .from('first')
  .union(function() {
    this.select('*').from('last');
  })
  .toSQL();

console.log(sql);

this yields:

WITH "first" AS (
    SELECT
        "fixtime"
    FROM
        "tc_positions"
    ORDER BY
        "fixtime" ASC
    LIMIT ?
),
"last" AS (
    SELECT
        "fixtime"
    FROM
        "tc_positions"
    ORDER BY
        "fixtime" DESC
    LIMIT ?
)
SELECT
    *
FROM
    "first"
UNION
SELECT
    *
FROM
    "last"