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SQL Server Statement Date Format Lookup Method

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1. Examples of how to deal with date formats and find specific date formats in SQL Server.

Handling date formats and finding records in a specific date format is a common requirement in SQL Server.SQL Server provides a variety of functions and formatting options to handle and compare dates. The following is a detailed example showing how to find records based on a specific date format.

1.1 Scene Description

Suppose we have a file namedOrderstable that contains order information, including a table namedOrderDatewhich stores the date of the order. Now, we want to find all orders created after January 1, 2023 (excluding that day).

1.2 Table structure

CREATE TABLE Orders (  
    OrderID INT PRIMARY KEY,  
    OrderDate DATE,  
    CustomerName VARCHAR(100),  
    Amount DECIMAL(10, 2)  
);

1.3 Insertion of example data

INSERT INTO Orders (OrderID, OrderDate, CustomerName, Amount)  
VALUES   
(1, '2023-01-02', 'Alice', 100.00),  
(2, '2023-01-01', 'Bob', 150.00),  
(3, '2023-02-01', 'Charlie', 200.00),  
(4, '2022-12-31', 'David', 120.00);

1.4 Finding orders after a specific date

To find all orders created after January 1, 2023 (excluding that day), we can simply use the>operator to compare dates. Because theOrderDateThe columns are alreadyDATEtypes, so we can compare them directly without additional format conversion.

SELECT *  
FROM Orders  
WHERE OrderDate > '2023-01-01';

1.5 Notes

  • In this example, we're not dealing with the date format directly, because theOrderDateThe columns are alreadyDATEtype, and the values we compare are also of typeYYYY-MM-DDformat is given, which is one of the standard formats for dates and times in SQL Server and can be used directly for comparisons.
  • If our date data is stored as a string and the format is notYYYY-MM-DDThen we may need to use theCONVERTmaybeCASTfunction converts it toDATEtypes and then compare them.
  • When we need to display dates in a specific format (e.g., in query results), we can use theCONVERTmaybeFORMATfunction. Note, however, that theFORMATfunction may be slower than others in SQL Server because it offers more formatting options.

1.6 Example: Displaying dates in a specific format

If we want to start withYYYY-MM-DDformat to display the date (although this is usuallyDATEtype's default format), but assuming we have a date column of string type, we can do this:

SELECT
    OrderID,
    CONVERT(VARCHAR, OrderDate, 23) AS FormattedOrderDate, -- Assuming OrderDate is of type DATE, used here only as an example
    CustomerName.
    Amount
FROM Orders
WHERE OrderDate > '2023-01-01'.

Note: In the aboveCONVERTExample.23is the style code to specify theYYYY-MM-DDformat. Note, however, that if theOrderDatealready areDATEtype, it is sufficient to select it directly, as SQL Server defaults to the type ofYYYY-MM-DDformat displayDATEThe value of the type.

Hopefully, this example will help us understand how to deal with date formatting and finding records with a specific date range in SQL Server.

2. How to find records in a specific date format in SQL Server

Finding records in a specific date format in SQL Server usually doesn't require any direct concern for the storage format because SQL Server'sDATE、DATETIME、DATETIME2、SMALLDATETIMEDate-time types such as these are not directly stored internally in some visible format (such as theYYYY-MM-DD) storage. These types are stored in a binary format and they allow SQL Server to perform valid date and time calculations.

However, when we retrieve these date-time type values from the database, SQL Server displays them in the default (or specified) format. However, during a query, we don't need (and shouldn't) filter records based on these display formats. Instead, we should use the date values themselves for comparison.

If we want to find records with a specific date (rather than format), we can compare them directly using date values. Here's an example showing how to find records created after a specific date (not including that day):

-- Assume the Orders table has an OrderDate column of type DATE or DATETIME.
SELECT *
FROM Orders
WHERE OrderDate > '2023-01-01'; -- Compare directly using date values

However, if we have date values stored in the database as strings (which is usually not the recommended practice as it can lead to type mismatches, sorting errors and performance issues) and we do need to look up the records in a specific string format, then we need to convert that string to a date type before comparing them. This can be done with theCONVERTmaybeCASTfunction to implement it:

-- Assuming that the OrderDate column is of type VARCHAR and that the stored date format is 'YYYY-MM-DD'
SELECT *
FROM Orders
WHERE CONVERT(DATE, OrderDate, 120) > '2023-01-01'; -- use CONVERT to convert the string to DATE type
-- or use CAST (if the format is always compatible with 120)
-- WHERE CAST(OrderDate AS DATE) > '2023-01-01'.

Note: In the aboveCONVERTExample.120is the style code that specifies that the input string is formatted as aYYYY-MM-DD HH:MI:SS(or justYYYY-MM-DDBecauseCONVERT(The time part is ignored when converting to a date). However, since we only care about the date part and assume that the input string will always contain only dates, even if the time part is ignored, using the120It is also safe to use as a style code. However, if the string format may be different, we should use a style code that matches our data.

Also, note that if theOrderDateThe columns are indeedVARCHARtype, and the stored date format is notYYYY-MM-DD, then we need to use a style code that matches our data (or not use it at all)CONVERTstyle parameter, but instead use a function or method that can handle a different format), and make sure that strings are correctly converted to date types before being compared. However, it is best practice to store datetime data in appropriate datetime type columns to avoid such problems.

3. How SQL Server stores date and time values

SQL Server uses several data types to store date and time values. The following are some of the date and time data types commonly used in SQL Server:

（1）DATE: Stores only date values (year, month, day) and contains no time information. The format is usually YYYY-MM-DD.

（2）TIME: Only time values (hours, minutes, seconds and optionally fractional seconds) are stored, no date information is included. Accuracy can be from 0 to 7 (number of digits in the fractional seconds section).

（3）DATETIME: Stores date and time values. Accuracy is fixed to 0.003 seconds (i.e. 3.33 milliseconds). The range is from 1/1/1753 to 12/31/999.

（4）DATETIME2: Stores date and time values with greater precision (up to 100 nanoseconds) and a larger date range (from 1/1/0001 to 12/31/9999). You can specify the precision of the fractional seconds portion (0 to 7).

（5）SMALLDATETIME: is a smaller version of DATETIME, with a smaller storage size and lower precision (minutes). The range is also from 1/1/1900 to 6/6/2079.

（6）DATETIMEOFFSET: Stores time zone information in addition to the date and time. This is particularly useful for storing dates and times that span multiple time zones. It is also possible to specify the precision of the fractional seconds part (0 to 7).

While creating a table in SQL Server, we can select the appropriate date and time data type to store the columns as required. For example:

CREATE TABLE Orders (  
    OrderID INT PRIMARY KEY,  
    OrderDate DATE,  
    OrderTime TIME(7),  
    OrderDateTime DATETIME2(3),  
    OrderSmallDateTime SMALLDATETIME,  
    OrderDateTimeOffset DATETIMEOFFSET(2)  
);

In this example, theOrderDate column usageDATE type to store only the date of the order.OrderTime column usageTIME(7) type to store time values accurate to 100 nanoseconds.OrderDateTime column usageDATETIME2(3) type to store dates and times with millisecond precision (although 3 decimal seconds are specified here, DATETIME2 can actually be more precise).OrderSmallDateTime column usageSMALLDATETIME type to store dates and times, but with lower precision and smaller ranges.OrderDateTimeOffset column usageDATETIMEOFFSET(2) type to store dates and times with time zone information with minute precision (since 2 decimal seconds are specified, but the actual precision of DATETIMEOFFSET can be higher, mainly for example purposes here).

When we insert data into these columns, SQL Server automatically converts the string or other type of value to the appropriate date and time type (if possible) or throws an error (if the conversion fails). When querying these columns, SQL Server returns the values in the standard date and time format, but we can also use theCONVERT maybeFORMAT function to customize the display format.

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