I don't understand the declare blocks. They are unnecessary and I think the above query has some issues.

Since you need 6 columns only, please check this query. I am not sure this is what you need.

SELECT 
    a.dist_code_combination_id var1,
    a.def_acctg_accrual_ccid var2,
    a.invoice_id,
    b.column4,
    (SELECT segment FROM gl_code_combinations WHERE code_combination_id = a.dist_code_combination_id) AS segment1,
    (SELECT segment FROM gl_code_combinations WHERE code_combination_id = a.def_acctg_accrual_ccid) AS segment2
FROM 
    AP_invoice_distributions_all a
JOIN 
    table2 b 
ON 
    a.invoice_id = b.column4;

I just thought you want to rename a.dist_code_combination_id to var1...