在PostgreSQL的查询中替换SELECT返回值

Question

I've got an issue to solve in a postgres model query.

The down below query returns me a table with three columns: "central", "imsi" and "mapver". Sometimes, the query will return blank values in "mapver", but i can't let a blank space exist into it. | How can i make this query substitute the blanks by "-" or a word, like "WrongBlank"? It is a varchar field.

SELECT Test_Configs.central, Test_Configs.imsi, 
       Test_Configs.mapver      

FROM config_imsis_centrais AS Default_Configs              -- Valores padrão da central correta
    LEFT JOIN config_imsis_centrais AS Test_Configs        -- Valores das centrais a serem testadas
        ON Default_Configs.central = 'ZBLM04'
        AND Default_Configs.ts = (SELECT MAX(ts) FROM config_imsis_centrais)
        AND Default_Configs.imsi = Test_Configs.imsi
        AND Default_Configs.ts = Test_Configs.ts
        AND Test_Configs.central <> Default_Configs.central
WHERE (                                                    -- Análise:
            COALESCE(Default_Configs.mapver, 'null') <> COALESCE(Test_Configs.mapver, 'null') AND
            Test_Configs.central <> ''
       )

A more simple example... I often get:

central  |   imsi   |   mapver
--------------------------------
ZSPO03   |   74402  |   
ZSPO03   |   74401  |   
ZSPO03   |   72434  |   
ZSPO03   |   72415  |

But I want:

central  |   imsi   |   mapver
--------------------------------
ZSPO03   |   74402  |     -
ZSPO03   |   74401  |     -
ZSPO03   |   72434  |     -
ZSPO03   |   72415  |     -

Thanks a lot!

Answer 1

http://www.postgresql.org/docs/current/static/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL

The COALESCE function returns the first of its arguments that is not null. Null is returned only if all arguments are null. It is often used to substitute a default value for null values when data is retrieved for display, for example:

SELECT COALESCE(description, short_description, '(none)')

A text constant can never be null. So, if the things before it are null, the text string will be returned.

SELECT Test_Configs.central, Test_Configs.imsi, 
       COALESCE(Test_Configs.mapver, 'whatever string you want')      

FROM config_imsis_centrais AS Default_Configs              -- Valores padrão da central correta
    LEFT JOIN config_imsis_centrais AS Test_Configs        -- Valores das centrais a serem testadas
        ON Default_Configs.central = 'ZBLM04'
        AND Default_Configs.ts = (SELECT MAX(ts) FROM config_imsis_centrais)
        AND Default_Configs.imsi = Test_Configs.imsi
        AND Default_Configs.ts = Test_Configs.ts
        AND Test_Configs.central <> Default_Configs.central
WHERE (                                                    -- Análise:
            COALESCE(Default_Configs.mapver, 'null') <> COALESCE(Test_Configs.mapver, 'null') AND
            Test_Configs.central <> ''
       )

EDIT, has empty strings, not nulls

Reference: http://www.postgresql.org/docs/9.3/static/functions-conditional.html#FUNCTIONS-CASE

SELECT Test_Configs.central, Test_Configs.imsi, 
       CASE Test_Configs.mapver WHEN '' THEN '-'
           ELSE COALESCE(Test_Configs.mapver, '-') 
       END AS mapver  

FROM config_imsis_centrais AS Default_Configs              -- Valores padrão da central correta
    LEFT JOIN config_imsis_centrais AS Test_Configs        -- Valores das centrais a serem testadas
        ON Default_Configs.central = 'ZBLM04'
        AND Default_Configs.ts = (SELECT MAX(ts) FROM config_imsis_centrais)
        AND Default_Configs.imsi = Test_Configs.imsi
        AND Default_Configs.ts = Test_Configs.ts
        AND Test_Configs.central <> Default_Configs.central
WHERE (                                                    -- Análise:
            COALESCE(Default_Configs.mapver, 'null') <> COALESCE(Test_Configs.mapver, 'null') AND
            Test_Configs.central <> ''
       )