节点mysql如果select获得结果，则返回结果，否则执行不同的select并返回这些结果

Question

I have 2 statements that work fine alone when talking to MySQL db. Both stored in variables like aSQL and bSQL.

So i'm trying to do the following where i return aSQL if any records from the query exist, else return the results from bSQL.

let sql = `
IF (${aSQL}) THEN
BEGIN
    ${aSQL};
END;
ELSE
BEGIN
    ${bSQL};
END;
END IF;
`

I also tried the following from another SO post (replaced WHERE with AND because my query already has a WHERE):

let sql = `
with temp as (${aSQL})
select * from temp
union all
${bSQL} AND (select count(*) from  temp) =0
`

And even:

let sql = `SELECT IF(EXISTS(${aSQL}), 1, 0) ELSE ${bSQL}`

No mater what, i get You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near .

But if i simply do connection.query(aSQL) or connection.query(bSQL) it works.

How do you do this in a single query without permanently storing a procedure in the db?

Note for non-js devs sql = `${aSQL}` is just a template and inputs full string value of var aSQL that might be equal to SELECT * FROM db.table WHERE id=60 AND other=foo .

Answer 1

Found the solution here :

let sql = `${aSQL} UNION ALL ${bSQL} AND NOT EXISTS (${aSQL})`

Where aSQL and bSQL look similar to the following:

SELECT * FROM db.table WHERE id=60 AND other=foo
SELECT * FROM db.table WHERE id=47 AND other=bar

Answer 2

just wait for result, and if result is empty, execute next query:

connection.query(aSQL, function (err, result, fields) {
    if (err || result == 0 || result.length == 0) {
      connection.query(bSQL, function(err, result, fields){console.log(result)});
    }else {
      console.log(result)
    }    
 });