Question

我在mysql表中有一堆列，其名称以'X_'开头（例如：X_N，X_Pad，X_Server等）。现在，这些列可以为null。

我想找出哪个以'X_'开头的列最不是NULL。

Answer 1

COUNT(expr)

返回行中expr的非NULL值的数量   通过SELECT语句检索。

以下查询将返回每列的非空计数：

SELECT
COUNT(*)     AS Total,
COUNT(X_N)   AS NNC_N,
COUNT(X_Pad) AS NNC_Pad
FROM table;

您可以使用此查询从表中获取匹配列的列表：

SELECT COLUMN_NAME
FROM  INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = '<database_name>'
AND   TABLE_NAME = '<table_name>'
AND   COLUMN_NAME LIKE 'x\_%';

-- Output:
-- X_N
-- X_Pad
-- X_Server

您可以使用此查询构建查询：

SELECT CONCAT('SELECT ', GROUP_CONCAT('COUNT(`', COLUMN_NAME, '`) AS `NNC of ', COLUMN_NAME, '`'), ' FROM `', TABLE_NAME, '`')
FROM  INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = '<database_name>'
AND   TABLE_NAME = '<table_name>'
AND   COLUMN_NAME LIKE 'x\_%';

-- Output (added whitespace for readability):
-- SELECT
-- COUNT(`X_N`)      AS `NNC of X_N`,
-- COUNT(`X_Pad`)    AS `NNC of X_Pad`,
-- COUNT(`X_Server`) AS `NNC of X_Server`
-- FROM `<table_name>`

替代：

SELECT GROUP_CONCAT('SELECT \'', COLUMN_NAME, '\' AS `Col`, COUNT(`', COLUMN_NAME , '`) AS `NNC` FROM `', TABLE_NAME , '`' SEPARATOR ' UNION ALL ')
FROM  INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = '<database_name>'
AND   TABLE_NAME = '<table_name>'
AND   COLUMN_NAME LIKE 'x\_%';

-- Output (added whitespace for readability):
-- SELECT 'X_N'      AS `Col`, COUNT(`X_N`)      AS `NNC` FROM `<table_name>` UNION ALL 
-- SELECT 'X_Pad'    AS `Col`, COUNT(`X_Pad`)    AS `NNC` FROM `<table_name>` UNION ALL 
-- SELECT 'X_Server' AS `Col`, COUNT(`X_Server`) AS `NNC` FROM `<table_name>`

Answer 2

从db.tbl中选择count（X_N），count（X_Pad），count（X_Server）;

可以。

如果您只需要一种方法来生成此SQL，则可以使用

SELECT CONCAT("select ", GROUP_CONCAT("count(",column_name, ")"), " from ", table_schema, '.', table_name)
FROM information_schema.columns
WHERE table_schema='db' AND table_name='tbl' AND column_name LIKE 'X\_%';

Answer 3

也许这样的事情？（另）

SELECT 
(SELECT COUNT(id) FROM My_Table WHERE X_1 != NULL) as x1,
(SELECT COUNT(id) FROM My_Table WHERE X_2 != NULL) as x2,
(SELECT COUNT(id) FROM My_Table WHERE X_3 != NULL) as x3,
(SELECT COUNT(id) FROM My_Table WHERE X_4 != NULL) as x4;

如何在mysql中查找列不为空的次数？

3 个答案: