Problem Solution Discussion Using TIMESTAMP Values

... WHERE MONTHd = MONTHCURDATE AND DAYOFMONTHd = DAYOFMONTHCURDATE ; This kind of query com m only is applied t o biographical dat a t o find list s of act ors, polit icians, m usicians, et c., who were born on a part icular day of t he year. I t s t em pt ing t o use DAYOFYEAR t o solve on t his day problem s, because it result s in sim pler queries. But DAYOFYEAR doesnt work properly for leap years. The presence of February 29 t hrows off t he values for days from March t hrough Decem ber. • Who has a birt hday t his m ont h? I n t his case, it s necessary t o check only t he m ont h: ... WHERE MONTHd = MONTHCURDATE ; • Who has a birt hday next m ont h? The t rick here is t hat you cant j ust add one t o t he current m ont h t o get t he m ont h num ber t hat qualifying dat es m ust m at ch. That gives you 13 for dat es in Decem ber. To m ake sure you get 1 January , use eit her of t he following t echniques: • ... WHERE MONTHd = MONTHDATE_ADDCURDATE ,INTERVAL 1 MONTH; ... WHERE MONTHd = MODMONTHCURDATE ,12+1;

5.32 Using TIMESTAMP Values

5.32.1 Problem

You want a records creat ion t im e or last m odificat ion t im e t o be aut om at ically recorded.

5.32.2 Solution

The TIMESTAMP colum n t ype can be used for t his. However, it has propert ies t hat som et im es surprise people, so read t his sect ion t o m ake sure you know what youll be get t ing. Then read t he next few sect ions for som e applicat ions of TIMESTAMP colum ns.

5.32.3 Discussion

MySQL support s a TIMESTAMP colum n t ype t hat in m any ways can be t reat ed t he sam e way as t he DATETIME t ype. However, t he TIMESTAMP t ype has som e special propert ies: • The first TIMESTAMP colum n in a t able is special at record-creat ion t im e: it s default value is t he current dat e and t im e. This m eans you need not specify it s value at all in an INSERT st at em ent if you want t he colum n set t o t he records creat ion t im e; MySQL will init ialize it aut om at ically. This also occurs if you set t he colum n t o NULL when creat ing t he record. • The first TIMESTAMP is also special whenever any colum ns in a row are changed from t heir current values. MySQL aut om at ically updat es it s value t o t he dat e and t im e at which t he change was m ade. Not e t hat t he updat e happens only if you act ually change a colum n value. Set t ing a colum n t o it s current value doesnt updat e t he TIMESTAMP . • Ot her TIMESTAMP colum ns in a t able are not special in t he sam e way as t he first one. Their default value is zero, not t he current dat e and t im e. Also, t heir value does not change aut om at ically when you m odify ot her colum ns; t o updat e t hem , you m ust change t hem yourself. • A TIMESTAMP colum n can be set t o t he current dat e and t im e at any t im e by set t ing it t o NULL . This is t rue for any TIMESTAMP colum n, not j ust t he first one. The TIMESTAMP propert ies t hat relat e t o record creat ion and m odificat ion m ake t his colum n t ype part icularly suit ed for cert ain kinds of problem s, such as aut om at ically recording t he t im es at which t able rows are insert ed or updat ed. On t he ot her hand, t here are ot her propert ies t hat can be som ew hat lim it ing: • TIMESTAMP values are represent ed in CCYYMMDDhhmmss form at , which isnt especially int uit ive or easy t o read, and oft en needs reform at t ing for display. • The range for TIMESTAMP values st art s at t he beginning of t he year 1970 and ext ends t o about 2037. I f you need a larger range, you need t o use DATETIME values. The following sect ions show how t o t ake advant age of t he TIMESTAMP t ypes special propert ies.

5.33 Recording a Rows Last Modification Time