You can provide a custom ordering, then take the first row, like this: Explanation: The inner select’s order by field(LOCALE, ‘ge’, ‘_’) gets you the rows in the order you define – in this case German first if it exists, then English (you could add more languages to the list). The “trick” here is using mysql’s “non-standard” GROUP … Read more
should work
I think what you are asking for will work by joining the Initial table to both Option_A and Option_B using LEFT JOIN, which will produce something like this: Example code: Once you have done this, you ‘ignore’ the set of NULLS. The additional trick here is in the SELECT line, where you need to decide … Read more
Taken from the MySQL manual (13.2.10.11 Rewriting Subqueries as Joins): A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better—a fact that is not specific to MySQL Server alone. So subqueries can be slower than LEFT [OUTER] JOIN, but in my opinion their strength … Read more
It looks as if you need an outer join, I’ll use a left one in my example: You can also leave out the outer keyword:
You should use & / | operators and be careful about operator precedence (== has lower precedence than bitwise AND and OR):
You should use & / | operators and be careful about operator precedence (== has lower precedence than bitwise AND and OR):
Simple example. Let’s select students with grades using left outer join: Now the same with left semi-join: The latter is much more efficient.
Your error on the snippet of data you posted is a little cryptic, in that because there are no common values, the join operation fails because the values don’t overlap it requires you to supply a suffix for the left and right hand side: merge works because it doesn’t have this restriction:
SQL supports qualifying a column by prefixing the reference with either the full table name: …or a table alias: The table alias is the recommended approach — why type more than you have to? Why Do These Queries Look Different? Secondly, my answers use ANSI-92 JOIN syntax (yours is ANSI-89). While they perform the same, … Read more