如何在PHP中合并两字段值并结合WHERE子句查询MySQL数据

Hey there! Let's get that query working properly for you. The issue with your original attempt is that MySQL doesn't use + for string concatenation—instead, it treats that as an arithmetic addition operator, which will give you totally unexpected results (like trying to add a number and a letter, which just ends up converting the letter to 0).

Here's the right way to write your query using MySQL's CONCAT() function, which is designed specifically for string concatenation:

SELECT * FROM table1 
WHERE CONCAT(USER, ID) = '100A' 
ORDER BY USER;

A few key notes to keep in mind: #

• Why CONCAT() instead of +? As mentioned, + does arithmetic in MySQL. If your USER field is a numeric type (like INT) and ID is a string, MySQL will try to cast ID to a number—since "A" can't be converted to a number, it becomes 0, so USER + ID would calculate to 100 + 0 = 100, which is not what you want. CONCAT() safely converts both values to strings and joins them together.
• Handling NULL values: If either USER or ID could be NULL, CONCAT() will return NULL for the entire expression, which won't match anything. To avoid this, use COALESCE() to replace NULL with an empty string:

  SELECT * FROM table1 
  WHERE CONCAT(COALESCE(USER, ''), COALESCE(ID, '')) = '100A' 
  ORDER BY USER;
  

• Performance tip: Using CONCAT() on fields in the WHERE clause means MySQL can't use any indexes on USER or ID for this condition. If you run this type of query often, consider adding a generated stored column to your table:

  ALTER TABLE table1 
  ADD COLUMN USER_ID VARCHAR(50) GENERATED ALWAYS AS (CONCAT(USER, ID)) STORED;
  

  Then create an index on this new column:

  CREATE INDEX idx_user_id ON table1(USER_ID);
  

  Now your query can be rewritten to use the indexed column for faster lookups:

  SELECT * FROM table1 
  WHERE USER_ID = '100A' 
  ORDER BY USER;
  

内容的提问来源于stack exchange，提问作者Raj