How to Efficiently Fetch Unique Rows with Maximum Date Values in SQL?

The only row with the maximum value in the SQL table with efficient extraction

SQL tables often include multiple lines with the same column values. When retrieved data from this type of table, it is important to extract the only line with the latest information or the maximum value of specific columns. This is especially useful in the continuous update of data.

Assume that a table called "MyTable" includes "Userid", "Value" and "Date". In order to retrieve the "userid" and "value" corresponding to each unique "userid" value, we can use an efficient SQL technology without relying on sub -query.

In this query, we use the left external connection between the two instances (T1 and T2) of "MyTable". The connection conditions ensure those lines that match the big "date" value in T1. Subsequently, the WHERE clause filter out all the lines with a matching line with a larger "Date" value in T2, leaving only the rows corresponding to the largest "date" value of each unique "userid".

<code class="language-sql">SELECT t1.*
FROM mytable t1
LEFT OUTER JOIN mytable t2
ON (t1.UserId = t2.UserId AND t1."Date" < t2."Date")
WHERE t2.UserId IS NULL;</code>

Another way of considering multiple lines with the same maximum date is:

In this variant, we use more fine connection conditions to process potential duplication with the same maximum "date" value. By introducing additional conditions (T1.ID & LT; T2.id), we can ensure that only the unique line with the minimum ID. (Assuming

<code class="language-sql">SELECT t1.*
FROM mytable t1
LEFT OUTER JOIN mytable t2
ON t1.UserId = t2.UserId AND ((t1."Date" < t2."Date") OR (t1."Date" = t2."Date" AND t1.id < t2.id))
WHERE t2.UserId IS NULL;</code>

is a primary key or unique identifier) ​​

id Through these methods, we can efficiently extract the unique row with the maximum date value from the SQL table, avoiding complex sub query, and improving the efficiency of query.

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