How does SQL Server determine key column order in missing index requests?
When SQL Server creates a missing index recommendation for a particular query plan, it separates possible key columns into 2 groups.  The first set contains all of the recommended columns that are part of an EQUALITY predicate.  The second set contains all of the recommended columns that are part of an INEQUALITY predicate.

Within each set, the columns are ordered by the ordinal position of the columns, based on the table definition.

(Many thanks to Brent Ozar for building a repro script against the Stack Overflow database to prove this!)

1. Create 3 identical tables, but put their columns in different order. (The reason here is to use a variety of column names and datatypes to show that that doesn't impact the column order in the missing index recommendation.)

CREATE TABLE dbo.NumberLetterDate (ID INT IDENTITY(1,1) PRIMARY KEY CLUSTERED, 
fINT INT, fNVARCHAR NVARCHAR(40), fDATE DATETIME, AboutMe NVARCHAR(MAX));
GO
CREATE TABLE dbo.LetterDateNumber (ID INT IDENTITY(1,1) PRIMARY KEY CLUSTERED, 
fNVARCHAR NVARCHAR(40), fDATE DATETIME, fINT INT, AboutMe NVARCHAR(MAX));
GO
CREATE TABLE dbo.DateNumberLetter (ID INT IDENTITY(1,1) PRIMARY KEY CLUSTERED,
fDATE DATETIME, fINT INT, fNVARCHAR NVARCHAR(40), AboutMe NVARCHAR(MAX));
GO


2. Populate the tables with the same data. Get 100,000 rows from the Users table with real-world data distribution.

INSERT INTO dbo.NumberLetterDate(fINT, fNVARCHAR, fDATE, AboutMe)
SELECT TOP 100000 Age, DisplayName, LastAccessDate, AboutMe
  FROM dbo.Users WITH (NOLOCK)
  ORDER BY Id;
GO
INSERT INTO dbo.LetterDateNumber(fINT, fNVARCHAR, fDATE, AboutMe)
SELECT TOP 100000 Age, DisplayName, LastAccessDate, AboutMe
  FROM dbo.Users WITH (NOLOCK)
  ORDER BY Id;
GO
INSERT INTO dbo.DateNumberLetter(fINT, fNVARCHAR, fDATE, AboutMe)
SELECT TOP 100000 Age, DisplayName, LastAccessDate, AboutMe
  FROM dbo.Users WITH (NOLOCK)
  ORDER BY Id;
GO


3. Write a query that needs an index. Start with 3 equality filters, filtering for an exact value in all 3 fields. Note that all 3 queries have the same fields in the same order:

SELECT ID
  FROM dbo.NumberLetterDate
  WHERE fINT = 100
  AND fNVARCHAR = 'Brent Ozar'
  AND fDATE = '2018/01/01'
  AND 1 = (SELECT 1);

SELECT ID
  FROM dbo.LetterDateNumber
  WHERE fINT = 100
  AND fNVARCHAR = 'Brent Ozar'
  AND fDATE = '2018/01/01'
  AND 1 = (SELECT 1);

SELECT ID
  FROM dbo.DateNumberLetter
  WHERE fINT = 100
  AND fNVARCHAR = 'Brent Ozar'
  AND fDATE = '2018/01/01'
  AND 1 = (SELECT 1);
GO


All three tables have exactly the same data, and the queries are identical. The only difference is the field order - and that's the difference in our missing index requests, too:



In the execution plans, the column order in the missing index request exactly matches the column order in the table. For example, in dbo.NumberLetterDate, the number column is first, so it's first in the missing index request, too:


On dbo.NumberLetterDate, the missing index is on fINT (number), fLetter (nvarchar), fDate, the same order of the fields in the table
On dbo.LetterDateNumber, the index order switches to fNVARCHAR, fDATE, fINT
On dbo.DateNumberLetter, the index order switches to fDATE, fINT, fNVARCHAR    


For a single-table operation like this, the index field order doesn't appear to depend on selectivity, datatype, or position in the query. (I leave it to other folks to prove this with more complex queries &amp; joins.)

4. Mix in an inequality filter. On the INT field, for example, put in &lt;> 100 as the filter:

SELECT ID
  FROM dbo.NumberLetterDate
  WHERE fINT &lt;&gt; 100
  AND fNVARCHAR = 'Brent Ozar'
  AND fDATE = '2018/01/01'
  AND 1 = (SELECT 1);

SELECT ID
  FROM dbo.LetterDateNumber
  WHERE fINT &lt;&gt; 100
  AND fNVARCHAR = 'Brent Ozar'
  AND fDATE = '2018/01/01'
  AND 1 = (SELECT 1);

SELECT ID
  FROM dbo.DateNumberLetter
  WHERE fINT &lt;&gt; 100
  AND fNVARCHAR = 'Brent Ozar'
  AND fDATE = '2018/01/01'
  AND 1 = (SELECT 1);
GO


In the execution plans, equality fields go first, then inequality fields - so here, fINT shows up last in all 3 missing index requests because it's an inequality search:



5. Use 3 inequality filters. Use the same search for all fields (&lt;>):

SELECT ID
  FROM dbo.NumberLetterDate
  WHERE fINT &lt;&gt; 100
  AND fNVARCHAR &lt;&gt; 'Brent Ozar'
  AND fDATE &lt;&gt; '2018/01/01'
  AND 1 = (SELECT 1);

SELECT ID
  FROM dbo.LetterDateNumber
  WHERE fINT &lt;&gt; 100
  AND fNVARCHAR &lt;&gt; 'Brent Ozar'
  AND fDATE &lt;&gt; '2018/01/01'
  AND 1 = (SELECT 1);

SELECT ID
  FROM dbo.DateNumberLetter
  WHERE fINT &lt;&gt; 100
  AND fNVARCHAR &lt;&gt; 'Brent Ozar'
  AND fDATE &lt;&gt; '2018/01/01'
  AND 1 = (SELECT 1);
GO


Since there's no equality searches, all 3 fields have the same priority order in the missing index recommendation, and now we're back to sorting purely by field order: