MySQL で select distinct の対象として多すぎるカラムを指定するとカラム数に比例して遅くなる。 そういった場合、一旦 pk を含む少数のカラムで distinct してから pk で引き直す方が早い。

Learn SQL basics in just 15 minutes with this tutorial focused on using SQL with MySQL. Topics covered include: - What SQL is ...

When querying data from a table, you may get duplicate rows. To remove these duplicate rows, you use the DISTINCT clause in the SELECT statement. Here’s the syntax of the DISTINCT clause: In this ...

I have a table that links two other tables together. I've defined an index on the first column, and its cardinality ends up being 218 (out of 5000+ rows). If I then ...