SQL Server进阶（三）单表查询

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一条完整的sql语句

select top | distinct   字段, 表达式, 函数, ...
from   表表达式
where  筛选条件
group by  分组条件
having     筛选条件
order by   排序依据;

SQL的执行顺序：

　　

运算符的优先级

SELECT语句的元素

SELECT empid, YEAR(orderdate) AS orderyear, COUNT(*) AS numorders
FROM Sales.Orders
WHERE custid = 71
GROUP BY empid, YEAR(orderdate)
HAVING COUNT(*) > 1
ORDER BY empid, orderyear;

FROM子句

WHERE子句

GROUP BY子句

SELECT
  empid,
  YEAR(orderdate) AS orderyear,
  SUM(freight) AS totalfreight,
  COUNT(*) AS numorders
FROM Sales.Orders
WHERE custid = 71
GROUP BY empid, YEAR(orderdate);

除了COUNT(*)之外，所有聚合函数忽略NULL标记。

DISTINCT去重

SELECT
  empid,
  YEAR(orderdate) AS orderyear,
  COUNT(DISTINCT custid) AS numcusts
FROM Sales.Orders
GROUP BY empid, YEAR(orderdate);

HAVING子句

　　HAVING语句通常与GROUP BY语句联合使用，用来过滤由GROUP BY语句返回的记录集。

　　HAVING语句的存在弥补了WHERE关键字不能与聚合函数联合使用的不足。

SELECT empid, YEAR(orderdate) AS orderyear
FROM Sales.Orders
WHERE custid = 71
GROUP BY empid, YEAR(orderdate)
HAVING COUNT(*) > 1;

SELECT子句

ORDER BY子句

SELECT empid, YEAR(orderdate) AS orderyear, COUNT(*) AS numorders
FROM Sales.Orders
WHERE custid = 71
GROUP BY empid, YEAR(orderdate)
HAVING COUNT(*) > 1
ORDER BY empid, orderyear;

TOP和OFFSET-FETCH筛选

PERCENT

SELECT TOP (1) PERCENT orderid, orderdate, custid, empid
FROM Sales.Orders
ORDER BY orderdate DESC;

WITH TIES

SELECT TOP (5) WITH TIES orderid, orderdate, custid, empid
FROM Sales.Orders
ORDER BY orderdate DESC;

OFFSET-FETCH筛选（跳过50取25）

SELECT orderid, orderdate, custid, empid
FROM Sales.Orders
ORDER BY orderdate, orderid
OFFSET 50 ROWS FETCH NEXT 25 ROWS ONLY;

开窗函数

SELECT orderid, custid, val,
  ROW_NUMBER() OVER(PARTITION BY custid
                    ORDER BY val) AS rownum
FROM Sales.OrderValues
ORDER BY custid, val;

谓词和运算符

CASE表达式

① 简单表达式：将一个值与一组可能的取值进行比较，并返回满足第一个匹配的结果；

SELECT productid,productname,categoryid,categoryname=(
    CASE categoryid
        WHEN 1 THEN 'Beverages'
        WHEN 2 THEN 'Condiments'
        WHEN 3 THEN 'Confections'
        WHEN 4 THEN 'Dairy Products'
        ELSE 'Unkonw Category'
    END)
FROM Production.Products;

② 搜索表达式：将返回结果为TRUE的第一个WHEN逻辑表达式所关联的THEN子句中指定的值。如果没有任何WHEN表达式结果为TRUE，CASE表达式则返回ELSE子句中出现的值。（如果没有指定ELSE，则默认返回NULL）；

SELECT orderid, custid, val, valuecategory=(
  CASE
    WHEN val < 1000.00    THEN 'Less than 1000'
    WHEN val BETWEEN 1000.00 AND 3000.00 THEN 'Between 1000 and 3000'
    WHEN val > 3000.00    THEN 'More than 3000'
    ELSE 'Unknown'
  END
)
FROM Sales.OrderValues

NULL标记

同时操作

使用字符数据

SELECT SUBSTRING('abcde', 1, 3); -- 'abc'

SELECT RIGHT('abcde', 3); -- 'cde'

SELECT LEN(N'abcde'); --

SELECT DATALENGTH(N'abcde'); --

SELECT CHARINDEX(' ','Itzik Ben-Gan'); --

SELECT PATINDEX('%[0-9]%', 'abcd123efgh'); --

SELECT REPLACE('1-a 2-b', '-', ':'); -- '1:a 2:b'

SELECT empid, lastname,
  LEN(lastname) - LEN(REPLACE(lastname, 'e', '')) AS numoccur
FROM HR.Employees;

SELECT REPLICATE('abc', 3); -- 'abcabcabc'

SELECT supplierid,
  RIGHT(REPLICATE('', 9) + CAST(supplierid AS VARCHAR(10)),
        10) AS strsupplierid
FROM Production.Suppliers;

SELECT STUFF('xyz', 2, 1, 'abc'); -- 'xabcz'

SELECT UPPER('Itzik Ben-Gan'); -- 'ITZIK BEN-GAN'

SELECT LOWER('Itzik Ben-Gan'); -- 'itzik ben-gan'

SELECT RTRIM(LTRIM('   abc   ')); -- 'abc'

SELECT FORMAT(1759, ''); -- '0000001759'

日期和时间

-- Current Date and Time
SELECT
  GETDATE()           AS [GETDATE],
  CURRENT_TIMESTAMP   AS [CURRENT_TIMESTAMP],
  GETUTCDATE()        AS [GETUTCDATE],
  SYSDATETIME()       AS [SYSDATETIME],
  SYSUTCDATETIME()    AS [SYSUTCDATETIME],
  SYSDATETIMEOFFSET() AS [SYSDATETIMEOFFSET];

SELECT
  CAST(SYSDATETIME() AS DATE) AS [current_date],
  CAST(SYSDATETIME() AS TIME) AS [current_time];

-- The CAST, CONVERT and PARSE Functions and their TRY_ Counterparts
SELECT CAST('' AS DATE);
SELECT CAST(SYSDATETIME() AS DATE);
SELECT CAST(SYSDATETIME() AS TIME);

SELECT CONVERT(CHAR(8), CURRENT_TIMESTAMP, 112);
SELECT CAST(CONVERT(CHAR(8), CURRENT_TIMESTAMP, 112) AS DATETIME);

SELECT CONVERT(CHAR(12), CURRENT_TIMESTAMP, 114);
SELECT CAST(CONVERT(CHAR(12), CURRENT_TIMESTAMP, 114) AS DATETIME);

SELECT PARSE('02/12/2007' AS DATETIME USING 'en-US');
SELECT PARSE('02/12/2007' AS DATETIME USING 'en-GB');

-- SWITCHOFFSET
SELECT SWITCHOFFSET(SYSDATETIMEOFFSET(), '-05:00');
SELECT SWITCHOFFSET(SYSDATETIMEOFFSET(), '+00:00');

-- TODATETIMEOFFSET
/*
UPDATE dbo.T1
  SET dto = TODATETIMEOFFSET(dt, theoffset);
*/

-- DATEADD
SELECT DATEADD(year, 1, '');

-- DATEDIFF
SELECT DATEDIFF(day, '', '');

SELECT
  DATEADD(
    day,
    DATEDIFF(day, '', CURRENT_TIMESTAMP), '');

SELECT
  DATEADD(
    month,
    DATEDIFF(month, '', CURRENT_TIMESTAMP), '');

SELECT
  DATEADD(
    month,
    DATEDIFF(month, '', CURRENT_TIMESTAMP), '');

-- DATEPART

SELECT DATEPART(month, '');

-- DAY, MONTH, YEAR

SELECT
  DAY('') AS theday,
  MONTH('') AS themonth,
  YEAR('') AS theyear;

-- DATENAME
SELECT DATENAME(month, '');

SELECT DATENAME(year, '');

-- ISDATE
SELECT ISDATE('');
SELECT ISDATE('');

-- fromparts
SELECT
  DATEFROMPARTS(2012, 02, 12),
  DATETIME2FROMPARTS(2012, 02, 12, 13, 30, 5, 1, 7),
  DATETIMEFROMPARTS(2012, 02, 12, 13, 30, 5, 997),
  DATETIMEOFFSETFROMPARTS(2012, 02, 12, 13, 30, 5, 1, -8, 0, 7),
  SMALLDATETIMEFROMPARTS(2012, 02, 12, 13, 30),
  TIMEFROMPARTS(13, 30, 5, 1, 7);

-- EOMONTH
SELECT EOMONTH(SYSDATETIME());

查询元数据

-- Catalog Views
USE TSQL2012;

SELECT SCHEMA_NAME(schema_id) AS table_schema_name, name AS table_name
FROM sys.tables;

SELECT
  name AS column_name,
  TYPE_NAME(system_type_id) AS column_type,
  max_length,
  collation_name,
  is_nullable
FROM sys.columns
WHERE object_id = OBJECT_ID(N'Sales.Orders');

-- Information Schema Views
SELECT TABLE_SCHEMA, TABLE_NAME
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_TYPE = N'BASE TABLE';

SELECT
  COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH,
  COLLATION_NAME, IS_NULLABLE
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = N'Sales'
  AND TABLE_NAME = N'Orders';

-- System Stored Procedures and Functions
EXEC sys.sp_tables;

EXEC sys.sp_help
  @objname = N'Sales.Orders';

EXEC sys.sp_columns
  @table_name = N'Orders',
  @table_owner = N'Sales';

EXEC sys.sp_helpconstraint
  @objname = N'Sales.Orders';

SELECT
  SERVERPROPERTY('ProductLevel');

SELECT
  DATABASEPROPERTYEX(N'TSQL2012', 'Collation');

SELECT
  OBJECTPROPERTY(OBJECT_ID(N'Sales.Orders'), 'TableHasPrimaryKey');

SELECT
  COLUMNPROPERTY(OBJECT_ID(N'Sales.Orders'), N'shipcountry', 'AllowsNull');