Mysql – Distribution of table in time

mysqlsql

I have a MySQL table with approximately 3000 rows per user. One of the columns is a datetime field, which is mutable, so the rows aren't in chronological order.

I'd like to visualize the time distribution in a chart, so I need a number of individual datapoints. 20 datapoints would be enough.

I could do this:

select timefield from entries where uid = ? order by timefield;

and look at every 150th row.

Or I could do 20 separate queries and use limit 1 and offset.

But there must be a more efficient solution…

Best Solution

Michal Sznajder almost had it, but you can't use column aliases in a WHERE clause in SQL. So you have to wrap it as a derived table. I tried this and it returns 20 rows:

SELECT * FROM (
    SELECT @rownum:=@rownum+1 AS rownum, e.*
    FROM (SELECT @rownum := 0) r, entries e) AS e2
WHERE uid = ? AND rownum % 150 = 0;

Syntax:

ALTER TABLE {TABLENAME} 
ADD {COLUMNNAME} {TYPE} {NULL|NOT NULL} 
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
WITH VALUES

Example:

ALTER TABLE SomeTable
        ADD SomeCol Bit NULL --Or NOT NULL.
 CONSTRAINT D_SomeTable_SomeCol --When Omitted a Default-Constraint Name is autogenerated.
    DEFAULT (0)--Optional Default-Constraint.
WITH VALUES --Add if Column is Nullable and you want the Default Value for Existing Records.

Notes:

Optional Constraint Name:
If you leave out CONSTRAINT D_SomeTable_SomeCol then SQL Server will autogenerate
a Default-Contraint with a funny Name like: DF__SomeTa__SomeC__4FB7FEF6

Optional With-Values Statement:
The WITH VALUES is only needed when your Column is Nullable
and you want the Default Value used for Existing Records.
If your Column is NOT NULL, then it will automatically use the Default Value
for all Existing Records, whether you specify WITH VALUES or not.

How Inserts work with a Default-Constraint:
If you insert a Record into SomeTable and do not Specify SomeCol's value, then it will Default to 0.
If you insert a Record and Specify SomeCol's value as NULL (and your column allows nulls),
then the Default-Constraint will not be used and NULL will be inserted as the Value.

Notes were based on everyone's great feedback below.
Special Thanks to:
@Yatrix, @WalterStabosz, @YahooSerious, and @StackMan for their Comments.

SQL update from one Table to another based on a ID match

I believe an UPDATE FROM with a JOIN will help:

MS SQL

UPDATE
    Sales_Import
SET
    Sales_Import.AccountNumber = RAN.AccountNumber
FROM
    Sales_Import SI
INNER JOIN
    RetrieveAccountNumber RAN
ON 
    SI.LeadID = RAN.LeadID;

MySQL and MariaDB

UPDATE
    Sales_Import SI,
    RetrieveAccountNumber RAN
SET
    SI.AccountNumber = RAN.AccountNumber
WHERE
    SI.LeadID = RAN.LeadID;