postgresql中使用distinct去重

select语法

[ WITH [ RECURSIVE ] with_query [, ...] ]
SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
    [ * | expression [ [ AS ] output_name ] [, ...] ]
    [ FROM from_item [, ...] ]
    [ WHERE condition ]
    [ GROUP BY grouping_element [, ...] ]
    [ HAVING condition [, ...] ]
    [ WINDOW window_name AS ( window_definition ) [, ...] ]
    [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ]
    [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
    [ LIMIT { count | ALL } ]
    [ OFFSET start [ ROW | ROWS ] ]
    [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
    [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF table_name [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]

where from_item can be one of:

    [ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
                [ TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed ) ] ]
    [ LATERAL ] ( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
    with_query_name [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
    [ LATERAL ] function_name ( [ argument [, ...] ] )
                [ WITH ORDINALITY ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
    [ LATERAL ] function_name ( [ argument [, ...] ] ) [ AS ] alias ( column_definition [, ...] )
    [ LATERAL ] function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
    [ LATERAL ] ROWS FROM( function_name ( [ argument [, ...] ] ) [ AS ( column_definition [, ...] ) ] [, ...] )
                [ WITH ORDINALITY ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
    from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]

and grouping_element can be one of:

    ( )
    expression
    ( expression [, ...] )
    ROLLUP ( { expression | ( expression [, ...] ) } [, ...] )
    CUBE ( { expression | ( expression [, ...] ) } [, ...] )
    GROUPING SETS ( grouping_element [, ...] )

and with_query is:

    with_query_name [ ( column_name [, ...] ) ] AS ( select | values | insert | update | delete )

TABLE [ ONLY ] table_name [ * ]

数据

INSERT INTO "test_dist" VALUES (1, '1', 'a');
INSERT INTO "test_dist" VALUES (2, '1', 'b');
INSERT INTO "test_dist" VALUES (3, '1', 'c');
INSERT INTO "test_dist" VALUES (4, '2', 'm');
INSERT INTO "test_dist" VALUES (5, '2', 'n');
INSERT INTO "test_dist" VALUES (6, '3', 'j');
INSERT INTO "test_dist" VALUES (7, '3', 'j');
INSERT INTO "test_dist" VALUES (8, '4', 'j');

去重多个列

直接用distinct，后面的列都参与去重。只有code, name拼接的组合相同时，去掉重复的

# SELECT 
DISTINCT code,name
from test_dist;
 code | name
------+------
 1    | b
 2    | n
 4    | j
 1    | c
 1    | a
 2    | m
 3    | j
(7 rows)

去重指定列，保留其他列

当下遇到需求，需要将其中一个列去重，然后其他列随机取出就可以了。造成这种需求的原因是单表设计不合理，没有拆分成多表，造成多字段冗余，除了唯一性标志外，其他字段是相同的。目标是，取出其他字段，忽略唯一标志。

因为其他字段有重复，需要去掉重复。

# SELECT 
DISTINCT ON (code) code,
id, name
from test_dist;
 code | id | name
------+----+------
 1    |  1 | a
 2    |  4 | m
 3    |  6 | j
 4    |  8 | j
(4 rows)

这里，根据code去重，id和name随机取出，这样可以获得code维度的数据。如果不去重，获得原始数据，code有重复。