多案例声明无法在Postgres中正常工作

Question

This is the query:

SELECT DISTINCT 
         completed_phases,
         CAST(completed_phases::bit(8) AS VARCHAR),
         CASE WHEN STRPOS(CAST(completed_phases::bit(8) AS VARCHAR),'1')=1 THEN 'FT' ELSE '' END ||
         CASE WHEN STRPOS(CAST(completed_phases::bit(8) AS VARCHAR),'1')=2 THEN 'ED' ELSE '' END ||
         CASE WHEN STRPOS(CAST(completed_phases::bit(8) AS VARCHAR),'1')=3 THEN 'MC' ELSE '' END ||
         CASE WHEN STRPOS(CAST(completed_phases::bit(8) AS VARCHAR),'1')=4 THEN 'HC' ELSE '' END ||
         CASE WHEN STRPOS(CAST(completed_phases::bit(8) AS VARCHAR),'1')=5 THEN 'UV' ELSE '' END ||
         CASE WHEN STRPOS(CAST(completed_phases::bit(8) AS VARCHAR),'1')=6 THEN 'TT' ELSE '' END ||
         CASE WHEN STRPOS(CAST(completed_phases::bit(8) AS VARCHAR),'1')=7 THEN 'RX' ELSE '' END ||
         CASE WHEN STRPOS(CAST(completed_phases::bit(8) AS VARCHAR),'1')=8 THEN 'PI' ELSE '' END
FROM rx_sales_order

If completed_phase is 129 , my output for final column should be FTPI. But it is only showing FT. Only the first case statement seems to work, even if all of them are distinct.

Answer 1

STRPOS() will always return the first occurance of the searched string. So all calls to strpos() will return 1 for the input value 129.

You can use substring() instead:

CASE WHEN substring(CAST(completed_phases::bit(8) AS VARCHAR),1,1)='1' THEN 'FT' ELSE '' END ||
CASE WHEN substring(CAST(completed_phases::bit(8) AS VARCHAR),2,1)='1' THEN 'ED' ELSE '' END ||
CASE WHEN substring(CAST(completed_phases::bit(8) AS VARCHAR),3,1)='1' THEN 'MC' ELSE '' END ||
CASE WHEN substring(CAST(completed_phases::bit(8) AS VARCHAR),4,1)='1' THEN 'HC' ELSE '' END ||
CASE WHEN substring(CAST(completed_phases::bit(8) AS VARCHAR),5,1)='1' THEN 'UV' ELSE '' END ||
CASE WHEN substring(CAST(completed_phases::bit(8) AS VARCHAR),6,1)='1' THEN 'TT' ELSE '' END ||
CASE WHEN substring(CAST(completed_phases::bit(8) AS VARCHAR),7,1)='1' THEN 'RX' ELSE '' END ||
CASE WHEN substring(CAST(completed_phases::bit(8) AS VARCHAR),8,1)='1' THEN 'PI' ELSE '' END

Another option would be to use get_bit() to test each bit individually:

case when get_bit(completed_phases::bit(8), 0) = 1 then 'FT' else '' END||
case when get_bit(completed_phases::bit(8), 1) = 1 then 'ED' else '' END||
case when get_bit(completed_phases::bit(8), 2) = 1 then 'MC' else '' END||
case when get_bit(completed_phases::bit(8), 3) = 1 then 'HC' else '' END||
case when get_bit(completed_phases::bit(8), 4) = 1 then 'UV' else '' END||
case when get_bit(completed_phases::bit(8), 5) = 1 then 'TT' else '' END||
case when get_bit(completed_phases::bit(8), 6) = 1 then 'RX' else '' END||
case when get_bit(completed_phases::bit(8), 7) = 1 then 'PI' else '' END

---

A more flexible way of doing that is to turn the bits into rows and use an array as a lookup. Something like:

with lookup (codes) as (
   values (array['FT','ED','MC','HC','UV','TT','RX','PI'])
)
SELECT  completed_phases, 
        completed_phases::bit(8),
        x.code
FROM rx_sales_order
  join lateral (
    select string_agg(codes[i],'') as code
    from lookup, unnest(string_to_array(completed_phases::bit(8)::text, null)) with ordinality as t(b,i) 
    where b = '1'
 ) as x on true

The part regexp_split_to_table(completed_phases::bit(8)::text, '') with ordinality as t(b,i) will return the following for the value 129:

b | i
--+--
1 | 1
0 | 2
0 | 3
0 | 4
0 | 5
0 | 6
0 | 7
1 | 8

code[i] the uses the index to lookup the matching code and string_agg() then puts all selected codes together again into a single string. The condition where b = '1' only selects the bits that are set.

That solution will be substantially slower than the hardcoded case expression (because it increases the number of rows, just to reduce them again) - but it is more flexible and easier to maintain.

---

If you need that a lot, the best option would be to put the case expression into a function and use the function in your queries.

create or replace function get_codes(p_phases integer)
  returns text
as 
$$
  select
    case when get_bit(p_phases::bit(8), 0) = 1 then 'FT' else '' END||
    case when get_bit(p_phases::bit(8), 1) = 1 then 'ED' else '' END||
    case when get_bit(p_phases::bit(8), 2) = 1 then 'MC' else '' END||
    case when get_bit(p_phases::bit(8), 3) = 1 then 'HC' else '' END||
    case when get_bit(p_phases::bit(8), 4) = 1 then 'UV' else '' END||
    case when get_bit(p_phases::bit(8), 5) = 1 then 'TT' else '' END||
    case when get_bit(p_phases::bit(8), 6) = 1 then 'RX' else '' END||
    case when get_bit(p_phases::bit(8), 7) = 1 then 'PI' else '' END
$$
language sql;

Then use:

SELECT DISTINCT 
         completed_phases,
         get_codes(completed_phases) as codes
FROM rx_sales_order

Answer 2

As pointed out in the answer by a_horse_with_no_name , strpos will return the first occurrence of the searched string. At any rate, it's better to use get_bit instead of casting to VARCHAR to check if a bit is set. Also, instead of || , you can use concat , which will handle null s as blank strings. Your query could then be rewritten to:

SELECT DISTINCT 
     completed_phases,
     CAST(completed_phases::bit(8) AS VARCHAR),
     concat(CASE get_bit(completed_phases::bit(8), 0) WHEN 1 THEN 'FT' END,
            CASE get_bit(completed_phases::bit(8), 1) WHEN 1 THEN 'ED' END,
            CASE get_bit(completed_phases::bit(8), 2) WHEN 1 THEN 'MC' END,
            CASE get_bit(completed_phases::bit(8), 3) WHEN 1 THEN 'HC' END,
            CASE get_bit(completed_phases::bit(8), 4) WHEN 1 THEN 'UV' END,
            CASE get_bit(completed_phases::bit(8), 5) WHEN 1 THEN 'TT' END,
            CASE get_bit(completed_phases::bit(8), 6) WHEN 1 THEN 'RX' END,
            CASE get_bit(completed_phases::bit(8), 7) WHEN 1 THEN 'PI' END)
FROM rx_sales_order;

On a side note, if you have the option to do so, I would recommend changing your database schema to store the phases as individual boolean columns instead of using a bit map. See Any disadvantages to bit flags in database columns? for a good discussion of why.