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Merge 表函数

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merge 表函数可让我们并行查询多个表。 它通过创建一个临时的 Merge 表来实现，并通过对这些表的列取并集以及推导通用类型来确定该表的结构。

设置表

我们将借助 Jeff Sackmann 的网球数据集 学习如何使用这个函数。 我们将处理包含自 20 世纪 60 年代以来比赛数据的 CSV 文件，不过会为每个年代创建略有不同的 schema。 我们还会为 20 世纪 90 年代额外添加几列。

导入语句如下所示：

CREATE OR REPLACE TABLE atp_matches_1960s ORDER BY tourney_id AS
SELECT tourney_id, surface, winner_name, loser_name, winner_seed, loser_seed, score
FROM url('https://raw.githubusercontent.com/JeffSackmann/tennis_atp/refs/heads/master/atp_matches_{1968..1969}.csv')
SETTINGS schema_inference_make_columns_nullable=0, 
         schema_inference_hints='winner_seed Nullable(String), loser_seed Nullable(UInt8)';

CREATE OR REPLACE TABLE atp_matches_1970s ORDER BY tourney_id AS 
SELECT tourney_id, surface, winner_name, loser_name, winner_seed, loser_seed, splitByWhitespace(score) AS score
FROM url('https://raw.githubusercontent.com/JeffSackmann/tennis_atp/refs/heads/master/atp_matches_{1970..1979}.csv')
SETTINGS schema_inference_make_columns_nullable=0, 
         schema_inference_hints='winner_seed Nullable(UInt8), loser_seed Nullable(UInt8)';

CREATE OR REPLACE TABLE atp_matches_1980s ORDER BY tourney_id AS
SELECT tourney_id, surface, winner_name, loser_name, winner_seed, loser_seed, splitByWhitespace(score) AS score
FROM url('https://raw.githubusercontent.com/JeffSackmann/tennis_atp/refs/heads/master/atp_matches_{1980..1989}.csv')
SETTINGS schema_inference_make_columns_nullable=0,
         schema_inference_hints='winner_seed Nullable(UInt16), loser_seed Nullable(UInt16)';

CREATE OR REPLACE TABLE atp_matches_1990s ORDER BY tourney_id AS
SELECT tourney_id, surface, winner_name, loser_name, winner_seed, loser_seed, splitByWhitespace(score) AS score,
       toBool(arrayExists(x -> position(x, 'W/O') > 0, score))::Nullable(bool) AS walkover,
       toBool(arrayExists(x -> position(x, 'RET') > 0, score))::Nullable(bool) AS retirement
FROM url('https://raw.githubusercontent.com/JeffSackmann/tennis_atp/refs/heads/master/atp_matches_{1990..1999}.csv')
SETTINGS schema_inference_make_columns_nullable=0,
         schema_inference_hints='winner_seed Nullable(UInt16), loser_seed Nullable(UInt16), surface Enum(\'Hard\', \'Grass\', \'Clay\', \'Carpet\')';

多个表的 schema

我们可以运行以下查询，并排列出每个表中的列及其类型，以便更直观地看出差异。

SELECT * EXCEPT(position) FROM (
    SELECT position, name,
       any(if(table = 'atp_matches_1960s', type, null)) AS 1960s,
       any(if(table = 'atp_matches_1970s', type, null)) AS 1970s,
       any(if(table = 'atp_matches_1980s', type, null)) AS 1980s,
       any(if(table = 'atp_matches_1990s', type, null)) AS 1990s
    FROM system.columns
    WHERE database = currentDatabase() AND table LIKE 'atp_matches%'
    GROUP BY ALL
    ORDER BY position ASC
)
SETTINGS output_format_pretty_max_value_width=25;
┌─name────────┬─1960s────────────┬─1970s───────────┬─1980s────────────┬─1990s─────────────────────┐
│ tourney_id  │ String           │ String          │ String           │ String                    │
│ surface     │ String           │ String          │ String           │ Enum8('Hard' = 1, 'Grass'⋯│
│ winner_name │ String           │ String          │ String           │ String                    │
│ loser_name  │ String           │ String          │ String           │ String                    │
│ winner_seed │ Nullable(String) │ Nullable(UInt8) │ Nullable(UInt16) │ Nullable(UInt16)          │
│ loser_seed  │ Nullable(UInt8)  │ Nullable(UInt8) │ Nullable(UInt16) │ Nullable(UInt16)          │
│ score       │ String           │ Array(String)   │ Array(String)    │ Array(String)             │
│ walkover    │ ᴺᵁᴸᴸ             │ ᴺᵁᴸᴸ            │ ᴺᵁᴸᴸ             │ Nullable(Bool)            │
│ retirement  │ ᴺᵁᴸᴸ             │ ᴺᵁᴸᴸ            │ ᴺᵁᴸᴸ             │ Nullable(Bool)            │
└─────────────┴──────────────────┴─────────────────┴──────────────────┴───────────────────────────┘

下面来看看这些差异：

  • 1970s 将 winner_seed 的类型从 Nullable(String) 改为 Nullable(UInt8)，并将 score 从 String 改为 Array(String)。
  • 1980s 将 winner_seed 和 loser_seed 的类型从 Nullable(UInt8) 改为 Nullable(UInt16)。
  • 1990s 将 surface 从 String 改为 Enum('Hard', 'Grass', 'Clay', 'Carpet')，并新增了 walkover 和 retirement 列。

使用 merge 查询多个表

让我们编写一个查询，找出 John McEnroe 战胜头号种子选手的比赛：

SELECT loser_name, score
FROM merge('atp_matches*')
WHERE winner_name = 'John McEnroe'
AND loser_seed = 1;
┌─loser_name────┬─score───────────────────────────┐
│ Bjorn Borg    │ ['6-3','6-4']                   │
│ Bjorn Borg    │ ['7-6','6-1','6-7','5-7','6-4'] │
│ Bjorn Borg    │ ['7-6','6-4']                   │
│ Bjorn Borg    │ ['4-6','7-6','7-6','6-4']       │
│ Jimmy Connors │ ['6-1','6-3']                   │
│ Ivan Lendl    │ ['6-2','4-6','6-3','6-7','7-6'] │
│ Ivan Lendl    │ ['6-3','3-6','6-3','7-6']       │
│ Ivan Lendl    │ ['6-1','6-3']                   │
│ Stefan Edberg │ ['6-2','6-3']                   │
│ Stefan Edberg │ ['7-6','6-2']                   │
│ Stefan Edberg │ ['6-2','6-2']                   │
│ Jakob Hlasek  │ ['6-3','7-6']                   │
└───────────────┴─────────────────────────────────┘

接下来，假设我们想对这些比赛记录进行过滤，找出其中 McEnroe 为 3 号种子或更低顺位的记录。 这会稍微麻烦一些，因为 winner_seed 在不同的表中使用了不同的类型：

SELECT loser_name, score, winner_seed
FROM merge('atp_matches*')
WHERE winner_name = 'John McEnroe'
AND loser_seed = 1
AND multiIf(
  variantType(winner_seed) = 'UInt8', variantElement(winner_seed, 'UInt8') >= 3,
  variantType(winner_seed) = 'UInt16', variantElement(winner_seed, 'UInt16') >= 3,
  variantElement(winner_seed, 'String')::UInt16 >= 3
);

我们使用 variantType 函数检查每一行中 winner_seed 的类型，再使用 variantElement 提取其底层值。 当类型为 String 时，我们会先将其转换为数值，再进行比较。 运行该查询后的结果如下所示：

┌─loser_name────┬─score─────────┬─winner_seed─┐
│ Bjorn Borg    │ ['6-3','6-4'] │ 3           │
│ Stefan Edberg │ ['6-2','6-3'] │ 6           │
│ Stefan Edberg │ ['7-6','6-2'] │ 4           │
│ Stefan Edberg │ ['6-2','6-2'] │ 7           │
└───────────────┴───────────────┴─────────────┘

使用 merge 时，这些行来自哪个表？

如果我们想知道这些行来自哪个表，可以怎么做？ 我们可以使用 _table 虚拟列，如下查询所示：

SELECT _table, loser_name, score, winner_seed
FROM merge('atp_matches*')
WHERE winner_name = 'John McEnroe'
AND loser_seed = 1
AND multiIf(
  variantType(winner_seed) = 'UInt8', variantElement(winner_seed, 'UInt8') >= 3,
  variantType(winner_seed) = 'UInt16', variantElement(winner_seed, 'UInt16') >= 3,
  variantElement(winner_seed, 'String')::UInt16 >= 3
);
┌─_table────────────┬─loser_name────┬─score─────────┬─winner_seed─┐
│ atp_matches_1970s │ Bjorn Borg    │ ['6-3','6-4'] │ 3           │
│ atp_matches_1980s │ Stefan Edberg │ ['6-2','6-3'] │ 6           │
│ atp_matches_1980s │ Stefan Edberg │ ['7-6','6-2'] │ 4           │
│ atp_matches_1980s │ Stefan Edberg │ ['6-2','6-2'] │ 7           │
└───────────────────┴───────────────┴───────────────┴─────────────┘

我们也可以在查询中使用这个虚拟列，统计 walkover 列各个值的数量：

SELECT _table, walkover, count()
FROM merge('atp_matches*')
GROUP BY ALL
ORDER BY _table;
┌─_table────────────┬─walkover─┬─count()─┐
│ atp_matches_1960s │ ᴺᵁᴸᴸ     │    7542 │
│ atp_matches_1970s │ ᴺᵁᴸᴸ     │   39165 │
│ atp_matches_1980s │ ᴺᵁᴸᴸ     │   36233 │
│ atp_matches_1990s │ true     │     128 │
│ atp_matches_1990s │ false    │   37022 │
└───────────────────┴──────────┴─────────┘

可以看到，除 atp_matches_1990s 外，其余所有数据的 walkover 列都为 NULL。 如果 walkover 列为 NULL，我们需要更新查询，检查 score 列是否包含字符串 W/O：

SELECT _table,
   multiIf(
     walkover IS NOT NULL,
     walkover,
     variantType(score) = 'Array(String)',
     toBool(arrayExists(
        x -> position(x, 'W/O') > 0,
        variantElement(score, 'Array(String)')
     )),
     variantElement(score, 'String') LIKE '%W/O%'
   ),
   count()
FROM merge('atp_matches*')
GROUP BY ALL
ORDER BY _table;

如果 score 的底层类型是 Array(String)，我们就必须遍历整个数组来查找 W/O；而如果它的类型是 String，则只需在字符串中搜索 W/O。

┌─_table────────────┬─multiIf(isNo⋯, '%W/O%'))─┬─count()─┐
│ atp_matches_1960s │ true                     │     242 │
│ atp_matches_1960s │ false                    │    7300 │
│ atp_matches_1970s │ true                     │     422 │
│ atp_matches_1970s │ false                    │   38743 │
│ atp_matches_1980s │ true                     │      92 │
│ atp_matches_1980s │ false                    │   36141 │
│ atp_matches_1990s │ true                     │     128 │
│ atp_matches_1990s │ false                    │   37022 │
└───────────────────┴──────────────────────────┴─────────┘
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