with hst as (
  select hh.code srv, hh.Amount, hh.dat, hh.contract
  from history hh
    left join services srv on hh.code <> 0 and srv.id = hh.code
  where hh.id = 123
),
rules as (
 select first 1 h.*
 from hst h
  left join DAYS_OFF df on df.DAT = h.dat
)
select first 1 h.Amount, h.srv
 from rules h
  left join contract_cache cn 
    left join contract_detail cd on 
       (h.srv <> 0 and cd.contract = cn.contract and cd.service = h.srv and cd.type >= 0)  
  on h.contract <> 0 and cn.id = h.contract

 select first 1 h.srv

 select first 1 h.code

 select first 1 h.*

7.5  <joined table>
***
            a) Each <column reference> directly contained in the <search
              condition> shall unambiguously reference a column of T1 or T2
              or be an  outer reference .

7.5  <joined table>
***
         <qualified join> ::=
              <table reference> [ NATURAL ] [ <join type> ] JOIN
                <table reference> [ <join specification> ]

         <join specification> ::=
                <join condition>
              | <named columns join>

         <join condition> ::= ON <search condition>
***

  21.2.9  COLUMNS view 
***
              FROM DEFINITION_SCHEMA.COLUMNS AS C
                  LEFT JOIN
                    DEFINITION_SCHEMA.DATA_TYPE_DESCRIPTOR AS D1
                      LEFT JOIN
                        DEFINITION_SCHEMA.COLLATIONS AS C1
                        ON
                        ( ( C1.COLLATION_CATALOG, C1.COLLATION_SCHEMA, C1.COLLATION_NAME )
                        = ( D1.COLLATION_CATALOG, D1.COLLATION_SCHEMA, D1.COLLATION_NAME ) )
                  ON
                    ( ( C.TABLE_CATALOG, C.TABLE_SCHEMA, C.TABLE_NAME, C.COLUMN_NAME )
                    = ( D1.TABLE_OR_DOMAIN_CATALOG, D1.TABLE_OR_DOMAIN_SCHEMA,
                          D1.TABLE_OR_DOMAIN_NAME, D1.COLUMN_NAME ) )
                  LEFT JOIN
                    DEFINITION_SCHEMA.DATA_TYPE_DESCRIPTOR AS D2
                      LEFT JOIN
                        DEFINITION_SCHEMA.COLLATIONS AS C2
                        ON
                        ( ( C2.COLLATION_CATALOG, C2.COLLATION_SCHEMA, C2.COLLATION_NAME )
                        = ( D2.COLLATION_CATALOG, D2.COLLATION_SCHEMA, D2.COLLATION_NAME ) )
                  ON*** и т.д. 

select first 1 h.Amount, h.srv
 from rules h
  left join contract_cache cn 
    on h.contract <> 0 and cn.id = h.contract
  left join contract_detail cd on 
    (h.srv <> 0 and cd.contract = cn.contract and cd.service = h.srv and cd.type >= 0)