从两个不同的表但相同的数据库中选择 SQL 查询

Question

i would like to select an sql query from two different tables. but both tables are in the same database.

this is my sql code and "air, temp, humidity, mq2" are from one table known as the "pi_sensors_network" while "Dust" is from another table known as "pi_dust_sensor". may i know how to go about selecting air, temp, humidity, mq2, Dust from 2 different tables but the same database? Thanks!

on a side note, there is no relation between both the tables. i just want to fetch the data from two different tables.

  $sql = "SELECT air, temp, humidity, mq2, Dust FROM pi_sensors_network ORDER BY time DESC LIMIT 1";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
     // output data of each row

Answer 1

Just use an alias for each table:

SELECT t1.air, t1.temp, t1.humidity, t1.mq2, t1.Dust As Dust1, t2.Dust As Dust2
FROM pi_sensors_network t1, 
pi_dust_sensor t2
ORDER BY time DESC LIMIT 1";

Answer 2

You left a few things out in your schema description and query. Do you have a field in one of the tables that refers to the equivalent record in the other table (ie a foreign key)? If so, you can use a JOIN. You didn't mention which table has the time field.

Here is my stab at your desired query, but making assumptions on the unanswered questions. I assumed that you had a field pi_sensors_network.id that is a primary key for the pi_sensors_network table. I also assumed that there was a field pi_dust_sensor.fkid that connected the rows in the pi_dust_sensor table with the rows in the pi_sensors_network table.

SELECT air, temp, humidity, mq2, Dust
FROM pi_sensors_network INNER JOIN
    pi_dust_sensor ON pi_sensors_network.id = pi_dust_sensor.fkid
ORDER BY time DESC LIMIT 1

Hopefully that will point you in the right direction. If you can answer my questions, I will be happy to update my answer.

Answer 3

you can combine data of two table in one object and can use after that how you want the output look like:

SELECT * ( SELECT air, temp, humidity, mq2, '' As Dust,[time] FROM pi_sensors_network union all select '' as air, '' as temp, '' as humidity, '' as mq2, Dust As Dust,[time] from pi_dust_sensor )base ORDER BY [TIME] DESC