根据条件和用户输入MySQL执行Select语句

Question

I am trying to allow a particular SELECT statement to execute based on a particular string sent to the IF condition. For Example, if the user selects support from the dropdown, a variable key is sent to the sql statement, and the appropriate SELECT Statement is executed.

DECLARE @ strg
SET @strg = 'input string'
IF(SELECT * FROM tickets WHERE ticket_section = @strg)
BEGIN
SELECT
usr_users.uname AS `user`,
support_chat_user.message AS user_message,
support_chat_user.created AS user_chat_date,
support_ticket.`subject` AS ticket_subject,
support_ticket.`status` AS ticket_state,
support_ticket.created AS ticket_date,
support_ticket.modified AS updated_date,
tickets.state
FROM
tickets
INNER JOIN support_chat_user ON support_chat_user.tickets_id = tickets.tick_id
INNER JOIN support_ticket ON support_ticket.tickets_id = tickets.tick_id
INNER JOIN usr_users ON support_chat_user.reporter_id = usr_users.uid
WHERE
tickets.tick_id = '9' AND
usr_users.uid = '1'
END

IF EXISTS(SELECT * FROM tickets WHERE ticket_section = @strg)

BEGIN
SELECT
usr_users.uname AS `user`,
support_chat_user.message AS user_message,
support_chat_user.created AS user_chat_date,
support_ticket.`subject` AS ticket_subject,
tickets.state
FROM
tickets
INNER JOIN support_chat_user ON support_chat_user.tickets_id = tickets.tick_id
INNER JOIN support_ticket ON support_ticket.tickets_id = tickets.tick_id
INNER JOIN usr_users ON support_chat_user.reporter_id = usr_users.uid
WHERE
tickets.tick_id = '9' AND
usr_users.uid = '1'

END

Answer 1

MySQL does not support IF, unless you put the code in a programming block. Your question is a bit hard to follow, because the text mentions one query but the code as two that seem to be identical.

You can move the conditional logic to the outermost WHERE clause in your query:

SELECT u.uname AS `user`, scu.message AS user_message, scu.created AS user_chat_date,
       st.`subject` AS ticket_subject, st.`status` AS ticket_state, st.created AS ticket_date, st.modified AS updated_date,
       t.state
FROM tickets t JOIN
     support_chat_user scu
     ON scu.tickets_id = t.tick_id JOIN
     support_ticket st
     ON st.tickets_id = t.tick_id JOIN
     usr_users u
     ON scu.reporter_id = u.uid
WHERE t.tick_id = 9 AND  -- presumably this is not numeric
      u.uid = 1 AND      -- presumably this is not numeric
      EXISTS (SELECT 1 FROM tickets t2 WHERE t2.ticket_section = @strg)

I'm not sure this is what you really want. I suspect you want:

WHERE t.tick_id = 9 AND  -- presumably this is not numeric
      u.uid = 1 AND      -- presumably this is not numeric
      (t.ticket_section = @strg OR @strg IS NULL)

However, that is merely informed speculation on what would be useful in the situation you describe.

Notes:

• Table aliases make the query easier to write and to read (as does proper indentation).
• Numeric constants should not be introduced with single quotes. I am guessing the ids are really numbers. If they are strings, then single quotes are appropriate.