Многие ко многим - примеры запросов

SELECT q.title, w.title FROM pages q, categories w, page_cat e WHERE q.id=e.idp AND w.id=e.idc AND wid IN (1,3)

DELETE FROM pages q WHERE q.id IN (SELECT idp FROM page_cat GROUP BY idp HAVING COUNT(id)>1)

CREATE TABLE IF NOT EXISTS page_cat (
    idp INTEGER NOT NULL,
    idc INTEGER NOT NULL,
    PRIMARY KEY(idp, idc)
);

SELECT p.id, GROUP_CONCAT(pc.idc) FROM pages p
LEFT JOIN page_cat pc ON p.id=pc.idp
GROUP BY p.id

SELECT p.id, GROUP_CONCAT(pc.idc) FROM pages p
LEFT JOIN page_cat pc ON p.id=pc.idp
WHERE p.id=1
GROUP BY p.id

DELETE FROM pages w WHERE w.id IN (SELECT q.idp FROM page_cat q WHERE q.idp IN (2,3) GROUP BY q.idp HAVING COUNT(q.id)>1)

CREATE TRIGGER delCategories
ON Categories
AFTER DELETE
AS
DELETE FROM page_cat 
WHERE idc = deleted.id
 
CREATE TRIGGER delPages
ON Pages
AFTER DELETE
AS
DELETE FROM page_cat 
WHERE idp = deleted.id

DELETE FROM pages WHERE id IN (2,3);
DELETE FROM page_cat WHERE idp IN (2,3);

DELETE FROM categoryes WHERE id_category IN (1,3);
DELETE FROM page_cat WHERE idc IN (1,3);

SELECT p.*, GROUP_CONCAT(c.id, ', ') id_category, GROUP_CONCAT(c.title, ', ') title_category
FROM pages p
LEFT JOIN page_cat pc ON p.id_page=pc.idp
LEFT JOIN categoryes c ON c.id=pc.idc
WHERE c.id=4
GROUP BY p.id
LIMIT 5

SELECT p.*, GROUP_CONCAT(c.id, ', ') id_category, GROUP_CONCAT(c.title, ', ') title_category
FROM (SELECT DISTINCT p1.* FROM page_cat pc1 JOIN pages p1 ON pc1.idp = p1.id WHERE pc1.idc = 4) p
LEFT JOIN page_cat pc ON p.id_page=pc.idp
LEFT JOIN categoryes c ON c.id=pc.idc
GROUP BY p.id
LIMIT 5