Either the article you read used a bad example, or you misinterpreted their point. select username from users where company = ‘bbc’ or company = ‘itv’; This is equivalent to: select username from users where company IN (‘bbc’, ‘itv’); MySQL can use an index on company for this query just fine. There’s no need to … Read more
UNION puts lines from queries after each other, while JOIN makes a cartesian product and subsets it — completely different operations. Trivial example of UNION: mysql> SELECT 23 AS bah -> UNION -> SELECT 45 AS bah; +—–+ | bah | +—–+ | 23 | | 45 | +—–+ 2 rows in set (0.00 sec) … Read more
(The following applies when every row is SQL DISTINCT, and outside SQL code similarly treats NULL like just another value.) Every base table has a statement template, aka predicate, parameterized by column names, by which we put a row in or leave it out. We can use a (standard predicate logic) shorthand for the predicate … Read more
You are trying to unpivot the data. MySQL does not have an unpivot function, so you will have to use a UNION ALL query to convert the columns into rows: select id, ‘a’ col, a value from yourtable union all select id, ‘b’ col, b value from yourtable union all select id, ‘c’ col, c … Read more
Here’s a plain implementation without using any third-party library. Main advantage over retainAll, removeAll and addAll is that these methods don’t modify the original lists input to the methods. public class Test { public static void main(String… args) throws Exception { List<String> list1 = new ArrayList<String>(Arrays.asList(“A”, “B”, “C”)); List<String> list2 = new ArrayList<String>(Arrays.asList(“B”, “C”, “D”, … Read more
UNION removes duplicate records (where all columns in the results are the same), UNION ALL does not. There is a performance hit when using UNION instead of UNION ALL, since the database server must do additional work to remove the duplicate rows, but usually you do not want the duplicates (especially when developing reports). To … Read more
You can do this either by a JOIN or Sub-query Sub-query documentation JOIN documentation