MySQL 8.0.1: [Recursive] Common Table Expressions in MySQL (CTEs), Part Four – depth-first or breadth-first traversal, transitive closure, cycle avoidance
I published a new CTE blog here.
WITH RECURSIVE and MySQL
[2017 update: MySQL 8.0.1 now features SQL-standard CTE syntax; more information is here ; the entry below, from 2013, shows how to work around the absence of CTEs in older MySQL versions.] If you have been using certain DBMSs, or reading recent versions of the SQL standard, you are probably aware of the so-called "WITH clause" of SQL. Some call it Subquery Factoring . Others call it Common Table Expression (CTE). In its simplest form, this feature is a kind of "boosted derived table". Assume that a table T1 has three columns: CREATE TABLE T1( YEAR INT, # 2000, 2001, 2002 ... MONTH INT, # January, February, ... SALES INT # how much we sold on that month of that year ); Now I want to know the sales trend (increase/decrease), year after year: SELECT D1.YEAR, (CASE WHEN D1.S>D2.S THEN 'INCREASE' ELSE 'DECREASE' END) AS TREND FROM (SELECT YEAR, SUM(SALES) AS S FROM T1 GROUP BY YEAR) AS D1, (SELECT YEAR, SUM(SALES) AS S FROM T1 GR
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