如何根据最新的 Date 和 Time OR Date_Time 列查询 SQL 的最新记录？

Question

Situation: SerialNumber has duplicate rows and I need to select the latest records and get what is the Status value (either it PASS or FAIL does not matter).

SerialNumber must be check based on the previous StationNumber. Example: I want to get the Status value of the latest record SerialNumber 197601234512345 in Station 2, so value StationNumber = @Station in the command line below is actually "Station 1"

Scenario: SerialNumber can either be pass or fail in Station 1, then SerialNumber exp:197601234512345 is brought forward to Station 2. In Station 2, need to check what latest Status of that SerialNumber exp:197601234512345 has in the previous StationNumber (Station 1).

Example table in the database db:

ID	SerialNumber	Date	Time	Date_Time	Status	StationNumber
213939	197601234512344	2021-04-28	11.53 AM	2021-04-28 11:53:00	PASS	Station 1
213940	197601234512345	2021-04-28	11.54 AM	2021-04-28 11:54:00	FAIL	Station 1
213941	197601234512345	2021-04-28	12.11 PM	2021-04-28 14:30:00	FAIL	Station 2
213942	197601234512345	2021-04-28	12.11 PM	2021-04-28 14:31:00	FAIL	Station 2
213943	197601234512344	2021-04-28	12.00 PM	2021-04-28 14:35:00	FAIL	Station 3
213944	197601234512344	2021-04-28	12.05 PM	2021-04-28 14:46:00	FAIL	Station 3
213945	197601234512344	2021-04-28	12.10 PM	2021-04-28 14:47:00	PASS	Station 3

Note: All datatype for the columns are VARCHAR(45)

So far, the SQL commands that I have tried has NO Error, but the MySqlreader does NOT Execute the ExecuteReader() which hold the SQL command. How do I know it? By doing the test display label on Label13.Text in Update 1: C# code, At the front-end system it display till Label13.Text = "Check 3"; then direct gives me Label13.Text = "Bad Output";

Below here are my five tried sql commands. But none of it works hurm =(:

Select Status from db.station where SerialNumber = @SN and StationNumber = @Station and MAX(CAST(Date_Time as DateTime))
or
Select Status from db.station where SerialNumber = @SN and StationNumber = @Station and MAX(CAST(Date_Time as DateTime)) Group By Status
or
Select Top Status from db.station where SerialNumber = @SN and StationNumber = @Station and MAX(CAST(Date_Time as DateTime))
or
Select Status from db.station where SerialNumber = @SN and StationNumber = @Station Oder By Status desc limit 1
or
Select a.Status from db.station INNER JOIN (Select SerialNumber, Status from db.station where SerialNumber = @SN and StationNumber = @Station and MAX(CAST(Date_Time as DateTime))group by SerialNumber) b on a.Status = b.Status

Update 1: C# code on 2021-04-29

if (textbox1.Text != "")
                    {
                        con.Open();
                        Label13.Text = "Check 1";

                        //new command
                        MySqlCommand cmdd = new MySqlCommand("Select SerialNumber, Status from db.station where SerialNumber = @SN  and StationNumber = @Station order by CAST(Date_Time as DateTime) desc limit 1 ", con);
                        
                        Label13.Text = "Check 2";
                        cmdd.Parameters.AddWithValue("@SN", textbox1.Text);
                        cmdd.Parameters.AddWithValue("@Station", "Station 1");
                        
                        Label13.Text = "Check 3";
                        MySqlDataReader read = cmdd.ExecuteReader();
                        Label13.Text = "Check 4"; 
                        if (read.HasRows)
                        {
                            
                            Label13.Text = "Check 5";
                            while (read.Read())
                            {
                                status = read.GetValue(1).ToString();
                                Label13.Text = status;

                                if (status == "PASS")
                                {
                                    con.Close();
                                    PreChecking.Text = "  Checked : Serial Number passed in previous Station 1"; 
                                    PreChecking.ForeColor = Color.Green;
                                    YourText1 = "hi";
                                    

                                    ////catch WO and CASING SN should be in here
                                    if (textbox1.Text != "")
                                    {
                                        //Auto Capture the Work Order when Entering the serial Number
                                        con.Open();
                                        MySqlCommand cmd = new MySqlCommand("Select WorkOrder from db.workorder where @Text  between coalesce (StartRange, @Text) and Coalesce (EndRange, @Text)", con);
                                        cmd.Parameters.AddWithValue("@Text", textbox1.Text);
                                        MySqlDataReader sdr = cmd.ExecuteReader();
                                        //if the serial number is in the work order range, it will display the work order, else display error message
                                        if (sdr.HasRows)
                                        {
                                            while (sdr.Read())
                                            {
                                                TextBox3.Text = sdr.GetValue(0).ToString();
                                                label12.Text = "  Checked: Valid W/O for Serial Number";
                                                label12.ForeColor = Color.Green;
                                                DropDownList3.Focus();
                                                Button1.Enabled = true;
                                                Button1.Focus();
                                                //**Close pop-up message as requested by Engineers**//
                                                //Page.ClientScript.RegisterStartupScript(this.GetType(), "popupscript", "alert(' Done checking Board Serial Number and validity W/O ');", true);

                                            }
                                        }
                                        else
                                        {
                                            TextBox3.Text = "";
                                            textbox1.Focus();
                                            label12.Text = "Alert: Invalid W/O for Serial Number";
                                            YourText4 = "Hi";
                                            label12.ForeColor = Color.Red;
                                            Button1.Enabled = false;
                                            Page.ClientScript.RegisterStartupScript(this.GetType(), "popupscript", "alert(' 1 Please aware of Invalid W/O for current scan Serial Number');", true);
                                        }
                                        con.Close();
                                    }
                                }
                                else if (status == "FAIL")
                                {
                                    
                                    PreChecking.Text = "  Alert: Serial Number not pass in previous Station 1";
                                    PreChecking.ForeColor = Color.Red;
                                    label12.Text = string.Empty;
                                    DropDownList2.Enabled = false;
                                    DropDownList3.Enabled = false;
                                    Button1.Enabled = false;
                                    Page.ClientScript.RegisterStartupScript(this.GetType(), "popupscript", "alert(' Serial NUmber cannot pass this station');", true);
                                }
                            }
                        }
                        else
                        {
                            Label13.Text = "Bad Output";
                        }

                    }

Hope anyone can help and spot the mistake or have a solution. Thank you.

Answer 1

You can use order by to get the latest row top and with Limit 1 you can select only the first row.

Select Status from db.station where SerialNumber = @SN and StationNumber = @Station order by CAST(Date_Time as DateTime) desc
limit 1

DB-Fiddle:

Schema and insert statements:

 create table station (SerialNumber varchar(50),    Date varchar(50),   Time varchar(50),   Date_Time varchar(50),  Status  varchar(50),StationNumber varchar(50));

 insert into station values('197601234512345'   ,'2021-04-28','11.53 AM'    ,'2021-04-28 11:53:00'  ,'PASS',    'Station 1');
 insert into station values('197601234512345'   ,'2021-04-28','11.54 AM',   '2021-04-28 11:54:00'   ,'FAIL',    'Station 1');
 insert into station values('197601234512345'   ,'2021-04-28','12.00 PM',   '2021-04-28 12:00:00'   ,'PASS',    'Station 1');
 insert into station values('197601234512345'   ,'2021-04-28','12.05 PM',   '2021-04-28 12:05:00'   ,'FAIL',    'Station 2');
 insert into station values('197601234512345'   ,'2021-04-28','12.10 PM',   '2021-04-28 12:10:00'   ,'PASS',    'Station 2');

Query:

 Select Status from station where SerialNumber ='197601234512345'  and StationNumber = 'Station 1' order by CAST(Date_Time as DateTime) desc
 limit 1

Output:

Status
PASS

db<fiddle here

Answer 2

To get last rows for all SerialNumber use:

MySQL 8+

WITH cte AS ( SELECT *, ROW_NUMBER() OVER (PARTITION BY SerialNumber ORDER BY Date_Time DESC) rn
              FROM db.station )
SELECT *
FROM cte
WHERE rn = 1;

MySQL 5+

SELECT db.station.*
FROM db.station
NATURAL JOIN ( SELECT SerialNumber, MAX(Date_Time) Date_Time
               FROM db.station
               GROUP BY SerialNumber ) maxdates

If you need these data for particular SerialNumber values only then add according WHERE clause.

If you need the data not for SerialNumber but for (SerialNumber, StationNumber) then modify the queries accordingly.

For single SerialNumber value - according WHERE and ORDER BY Date_Time DESC LIMIT 1 is enough.

Answer 3

Solution: my situation kinda relate to situation that happen in enter link description here

so, my new SQL command

SELECT a.ID, a.Date_Time, a.Status FROM db.station a, (SELECT MAX(ID) as id  FROM db.station WHERE SerialNumber = @SN Group by StationNumber = @Station) b WHERE a.ID = b.id

I test to insert Serial Number 197601234512345 in Station 3 finally, it gets the latest row from the previous Station (Station 2)

ID	SerialNumber	Date	Time	Date_Time	Status	StationNumber
213942	197601234512345	2021-04-28	12.11 PM	2021-04-28 14:31:00	PASS	Station 2