How to select only the first rows for each unique value of a column?

Step-it-up: Advanced applications and alternatives

Precise ordering: Multilevel sorting

You can add multiple sorting criteria in your ORDER BY clause. Excellent for those situations when the "first" row depends on more than one column:

ORDER BY primary_criteria, secondary_criteria DESC


This lets your SQL statement say "Ok, now we're getting serious!"

Old school: When window functions are not your friend

Dealing with an older database or SQL version without ROW_NUMBER()? No problem, let's go retro with GROUP BY and MIN():

SELECT your_table.*
FROM your_table
JOIN (
  SELECT unique_column, MIN(order_criteria) AS first_criteria
  FROM your_table
  GROUP BY unique_column
) AS subquery ON your_table.unique_column = subquery.unique_column
AND your_table.order_criteria = subquery.first_criteria;


Who said old-school isn't cool? 😎

Time travelers: Selecting rows based on insertion time

"First" row might mean the earliest inserted row. If that's your jam, use an inserted_time column, if available:

ORDER BY inserted_time ASC


Your SQL statement is now a time machine! 🕰

Speedy Gonzales: Considerations for performance

Remember that window functions on large datasets can pressure the SQL hamsters 🐹 running your server. Improve their lives by indexing the columns used in PARTITION BY and ORDER BY.

What if...?: Situations and scenarios

PostgreSQL and DISTINCT ON

If you're using PostgreSQL, you might like DISTINCT ON. It's like SQL's version of a clean shave:

SELECT DISTINCT ON (unique_column) *
FROM your_table
ORDER BY unique_column, order_criteria;


Data integrity: Fighting duplicates

Guard your table against sneakily-inserted duplicates that might confuse our "first row" logic with constraints or indexes.

JOINing the party: Handling complex conditions

Need to consider another table as part of your conditions? Fear not, SQL's got you covered with JOINS:

WITH RankedData AS (
  SELECT t1.*, t2.some_column,
    ROW_NUMBER() OVER (PARTITION BY t1.unique_column ORDER BY t1.order_criteria) AS rn
  FROM your_table t1
  JOIN other_table t2 ON t1.foreign_key = t2.primary_key
)
SELECT * FROM RankedData WHERE rn = 1;


It's like getting a VIP pass backstage! 🎉

Bells and whistles: Making your queries shine

A clean house: Selecting specific columns

Instead of *, select only the required columns. Your query runs faster and wins beauty contests. A real SQL superstar! 🌟

Pitfall prevention: Avoiding common traps

Beware of the hidden sinkholes with GROUP BY - improper use might result in mixing oranges and apples. Not a tasty SQL salad! 🍽

Extra spice: Combining aggregate functions with partitioning

Give your queries extra kick by combining aggregate functions with PARTITION BY:

SUM(column_name) OVER (PARTITION BY unique_column ORDER BY order_criteria)


This method lets you say to your data "Who's your aggregate function?"