Select item in dropdown needs to show / hide another dropdown
Show/Hide  dropdown, with jQuery, based on value
C1/D Visa - Mexico and US note: this question has been updated to reflect that we are currently using MySQL, having done so, I would like to see a how much easier it would be if we switched to a CTE-supporting database.  I have a self-referencing table with a primary key, id and a foreign key parent_id.  +------------+--------------+------+-----+---------+----------------+ | Field      | Type         | Null | Key | Default | Extra          | +------------+--------------+------+-----+---------+----------------+ | id         | int(11)      | NO   | PRI | NULL    | auto_increment |  | parent_id  | int(11)      | YES  |     | NULL    |                |  | name       | varchar(255) | YES  |     | NULL    |                |  | notes      | text         | YES  |     | NULL    |                |  +------------+--------------+------+-----+---------+----------------+   Given a name, how can I query the top-level parent?  Given a name, how can I query all of the id's associated with a record of name = 'foo'?  context: I am not a dba, but am planning to ask a dba to implement this type of hierarchical structure and would like to test some queries. The motivation for doing so is described by .    Here is an example of the relationships among ids in the table:    -- ----------------------------------------------------- -- Create a new database called 'testdb' -- ----------------------------------------------------- SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0; SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0; SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='TRADITIONAL';  CREATE SCHEMA IF NOT EXISTS `testdb` DEFAULT CHARACTER SET latin1 COLLATE latin1_swedish_ci ; USE `testdb` ;  -- ----------------------------------------------------- -- Table `testdb`.`observations` -- ----------------------------------------------------- CREATE  TABLE IF NOT EXISTS `testdb`.`observations` (   `id` INT NOT NULL ,   `parent_id` INT NULL ,   `name` VARCHAR(45) NULL ,   PRIMARY KEY (`id`) ) ENGINE = InnoDB;  SET SQL_MODE=@OLD_SQL_MODE; SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS; SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS;  -- ----------------------------------------------------- -- Add Example Data Set -- -----------------------------------------------------   INSERT INTO observations VALUES (1,3), (2,5), (3,NULL), (4,10),     (5,NULL), (6,1), (7,5), (8,10), (9,10), (10,3); How exactly does curved space-time describe the force of gravity? How to increase battery life on Ubuntu 20.04 and what power saving software should I install, if any? I recently installed Ubuntu 20.04 LTS on my Lenovo Ideapad Flex 5 and realized that battery life in Ubuntu is not as good as Windows. The battery drains faster in Ubuntu. I have seen the questions in Ask Ubuntu about this, but I haven't found anything that regards Ubuntu 20.04 LTS. I've done some research and found out that the battery drain is because most vendors optimize their drivers for Windows and with Linux, you have to hope that the kernel drivers do the job (which they usually do). After that, I researched how to increase my battery life and almost all sources I found recommended installing tlp or powertop or something similar to these. But I have a few questions.  Does tlp, powertop, and similar software actually affect battery life?  If the answer to 1 is yes (in a positive way), what would you recommend I install (such as whether I should  install tlp or powertop by itself or in a combination)?  Are there any other ways to increase battery life in Ubuntu 20.04?   Note: I am a beginner to Linux and Linux distros and do not want to mess with anything advanced that may negatively affect the system. I may not be able to immediately know how to tweak any of the aforementioned software, so please suggest any tweaks if they are needed to get results. And if it helps to know, here are my system specs: OS: Ubuntu 20.04.1 LTS x86_64 Host: 81X2 IdeaPad Flex 5 14ARE05 Kernel: 5.7.17-050717-generic CPU: AMD Ryzen 7 4700U with Radeon Graphics (8) @ 2.000GHz GPU: AMD ATI 04:00.0 Renoir (These were given by neofetch) P.S: If you need anything else, just ask. Thanks in advance! Design a custom Beamer theme from scratch I'm working on this problem from Elementary Analysis by Ross which is intuitive when sketched but keeps stymieing me when I try to write it out.  Let $f$ be a continuous function on $[a,b] \subset \mathbb{R}$. Define $f^\star (x)$ as:  $$ f^\star(x) = \sup \{f(y)\mid a \leq y \leq x \} $$ Prove that $f^\star$ is a continuous increasing function on $[a,b]$.  Things I've figured out: (these are mostly trivial)   $f$ is uniformly continuous on $[a,b]$. Since $[a,b]$ is closed and bounded there exists some $c \in [a,b]$ such that $f(c) \geq f(x)\ \forall x \in [a,b]$. (In words: the supremum is actually attained.) If $c \in [a,b]$ is as above, then $f^\star$ is constant (and hence continuous) on $[c,b]$ (which is possibly a singleton).   This seems completely obvious when you actually draw a continuous function but translating that to a formal proof eludes me... If a lens focuses all incoming light to a point, how do we get 2D images? How do lenses produce 2-dimensional images, if a lens bends all incoming rays of light to intersect at the focal point? Shouldn't this produce a single dot of light on a screen placed at the focal length?  This is basically the standard diagram that always shows up in textbooks:    I know this doesn't happen in real life--I used to use telescopes pretty frequently for work. The most in-focus image would be the one with the smallest diameters for the stars, which, of course, we think of point sources at infinity. But--the light from all stars (despite being at infinity) is not all focused at a single point. Instead, each star's light is focused at its own $(x,y)$ point on a 2D image.  I can't reconcile the theory as I understand it with any of my real-life experience with optics. What am I missing? Performing Point Distance analysis using Basic level license of ArcGIS Desktop? I am using ArcView (ArcGIS Desktop Basic) 10.1 and I need to perform a point distance analysis.   What are the steps to determine the distance from A to B-Z, B to A-Z, C to A-Z, etc? Can a multi-class Bard/Wizard write spells known from Bard, into Wizard spellbook? What happened to Rhaegal? In Game of Thrones S08E03 ("The Long Night"), the Night King's dragon tore at the one Jon Snow rides (Rhaegal). He falls off of it and we don't see the dragon anymore, is it dead? I'm quite confused about the following regular expressions I found in a shell script:  ${0##*/} ${0%/*}   How do they work? Migrated questions don't have a user action label in the question list Query to collect the recent TimeSampled row using inner joins in MS SQL Why PlotStyle->Dashed make everything dashed? Plurals of acronyms, letters, numbers — use an apostrophe or not? Let $A$ and $B$ be two real $n\times n$ matrices such that $AB-BA=A$ Prove that $A$ is not invertible and that $A$ is nilpotent.  My attempt is the following. It holds that $AB=(B+I)A$ If $A$ were invertible, $B=A^{-1}(B+I)A$. Taking trace on both sides yields $tr(B)=tr(B)+n$, hence $n=0$, which is a contradiction. I can't prove that $A$ is nilpotent though. I have a project that needs a different voltage (or multiple voltages) than what I have available. Sometimes I need DC instead of AC, or I need a lower or higher voltage. How do I convert from one to another? Should I use resistors or a regulator or something else?  I've heard of converters, transformers, regulators, inverters, step-down, step-up, buck, boost, switching, linear, and other terms. Or acronyms (initialisms) like PMIC and LDO. Can someone shed some light on all this? Direct Link to a Comment The increase in the character limit of comments has led them to become as important as the questions and answers that spawn them. On several occasions (especially on meta) I've found reason to want to link to a comment directly. I'd imagine something as simple as making the date / time stamp an anchored link which mimics the behavior of the question link. Add layer to a QGIS group using Python After beating the final boss, can I return to the original castle?