多个内连接 mysql

Question

I am writing an sql statement in which I have multiple INNER JOIN statements with a final WHERE clause. I have achieved this using the following query:

select weapon_name 
  from special_weapons
     , player
     , player_weapons 
 where player.uuid = player_weapons.uuid 
   and player_weapons.weaponid = special_weapons.weaponid;

However, this inner join syntax is considered bad practice. I have tried rewriting this query numerous times and continually run into a gambit of problems. I attempted the following, all of which failed:

SELECT sp1.WEAPON_NAME 
        FROM SPECIAL_WEAPONS sp1, PLAYER_WEAPONS pw 
    INNER JOIN PLAYER p1 ON 
        pw.UUID = p1.uuid 
    INNER JOIN PLAYER_WEAPONS pw1 ON 
        sp1.weaponid = pw1.weapondid;

ERROR 1054 (42S22): Unknown column 'sp1.weaponid' in 'on clause

Attempt 2:

SELECT WEAPON_NAME 
  FROM SPECIAL_WEAPONS, PLAYER_WEAPONS 
 INNER JOIN PLAYER ON PLAYER_WEAPONS.UUID = PLAYER.UUID 
 INNER JOIN PLAYER_WEAPONS ON SPECIAL_WEAPONS.WEAPONID = 
    PLAYER_WEAPONS.WEAPONID;

ERROR 1066 (42000): Not unique table/alias: 'PLAYER_WEAPONS'

Attempt 3:

SELECT WEAPON_NAME 
  FROM SPECIAL_WEAPONS, PLAYER_WEAPONS 
INNER JOIN PLAYER ON PLAYER_WEAPONS.UUID = PLAYER.UUID 
INNER JOIN PLAYER_WEAPONS pw ON SPECIAL_WEAPONS.WEAPONID = pw.WEAPONID;

ERROR 1054 (42S22): Unknown column 'SPECIAL_WEAPONS.WEAPONID' in 'on clause'

My tables look as follows:

What am I doing wrong?

Answer 1

try this

SELECT weapon_name
FROM player p
JOIN player_weapons pw ON p.uuid = pw.uuid
JOIN special_weapons sw ON pw.weaponid = sw.weaponid

there is no magic stuff behind this query. I just rewrite it using your original successful query. you had a hard time probably because the table order in the query.

Answer 2

What am I doing wrong?

You write that this inner join syntax is considered bad practice. This is true partially - comma-separated tables join is CROSS JOIN, not INNER (but in MySQL they're synonims - MySQL extension allows any JOIN type clause without ON clause rather than another DBMSs which allows this for CROSS JOIN only). The words bad practice are correct 100%.

But then you try to use the mix of comma-separated join and INNER join - this is not bad but extremely bad practice.

---

And detailed:

Attempt1.

SELECT sp1.WEAPON_NAME 
        FROM SPECIAL_WEAPONS sp1, PLAYER_WEAPONS pw 
    INNER JOIN PLAYER p1 ON 
        pw.UUID = p1.uuid 
    INNER JOIN PLAYER_WEAPONS pw1 ON 
        sp1.weaponid = pw1.weapondid;

means

SELECT sp1.WEAPON_NAME 
FROM SPECIAL_WEAPONS sp1, (            PLAYER_WEAPONS pw 
                            INNER JOIN PLAYER p1 ON pw.UUID = p1.uuid 
                            INNER JOIN PLAYER_WEAPONS pw1 ON sp1.weaponid = pw1.weapondid 
                          );

sp1 alias is not accessible within the parenthesis because this alias is defined out of them.

---

Attempt 2.

SELECT WEAPON_NAME 
  FROM SPECIAL_WEAPONS, PLAYER_WEAPONS                    -- first PLAYER_WEAPONS
 INNER JOIN PLAYER ON PLAYER_WEAPONS.UUID = PLAYER.UUID 
 INNER JOIN PLAYER_WEAPONS ON SPECIAL_WEAPONS.WEAPONID =  -- second PLAYER_WEAPONS
    PLAYER_WEAPONS.WEAPONID;

---

Attempt 3.

SELECT WEAPON_NAME 
  FROM SPECIAL_WEAPONS, PLAYER_WEAPONS 
INNER JOIN PLAYER ON PLAYER_WEAPONS.UUID = PLAYER.UUID 
INNER JOIN PLAYER_WEAPONS pw ON SPECIAL_WEAPONS.WEAPONID = pw.WEAPONID;

means

SELECT WEAPON_NAME 
  FROM SPECIAL_WEAPONS, (            PLAYER_WEAPONS 
                          INNER JOIN PLAYER ON PLAYER_WEAPONS.UUID = PLAYER.UUID 
                          INNER JOIN PLAYER_WEAPONS pw ON SPECIAL_WEAPONS.WEAPONID = pw.WEAPONID
                        );

Non-aliased SPECIAL_WEAPONS table copy is not accessible within the parenthesis.

Answer 3

Try this

SELECT SW.WEAPON_NAME FROM SPECIAL_WEAPONS SW 
INNER JOIN PLAYER_WEAPONS PW ON PW.WEAPONID = SW.WEAPONID 
INNER JOIN PLAYER P ON P.UUID = PW.UUID;

you can use the WHERE clause to get output of three table join.